The manuscript describes the effect of water on photocatalytic oxidation of methane to methanol. Although it is interesting to see that the oxidation was promoted significantly by the addition of water, the manuscript lacks sufficient experimental results to support the conclusion, especially the explanation on the role of water. Thus, the manuscript does not seem to meet the criteria for the publication in Chemical Science, in which new findings of exceptional significance with scientific reliability is required. The following issues should be addressed for instance.
1. The authors explained that the highly reduced Ag species formed upon an adsorption of water is responsible for the effective formation of C–O bond. I cannot find a result to support this explanation. DRIFTS only shows that C–O bond forms under photoirradiation in the presence of water and provides no evidence for the participation of the reduced Ag species. Moreover, Figure 3c shows that a population of the reduced Ag species is small compared with that of other Ag species after reaction even in the presence of water. Thus, the claim that the existence of those highly reduced Ag species is responsible for the high activity of Ag/InGaN in photocatalytic methane oxidation seems not reasonable.
2. The authors proposed that one of the roles of water is to promote the desorption of methanol from the theoretical calculation. This should be confirmed experimentally such as by DRIFTS.
3. The authors claimed that the electron transfer from InGaN to Ag nanoparticles takes place in the presence of water based on the result of PDOS in Figure 3e and 3f. I do not understand this. The PDOS plots only reflect the effect of water on the electronic structure of Ag or In and provide no information on the interaction between Ag and InGaN. Thus, there is no proof for the electron transfer. If the electron transfer takes place, what is the driving force?
4. The change in the PDOS plots was small. Thus, the PDOS cannot explain the promotion effect of water on this reaction.
5. ESR measurements using a spin trapping reagent such as DMPO should be carried out in the presence and absence of water to examine a change in the active species which can affect the efficiency and selectivity of the reaction.
6. Some basic control experiments are required such as a reaction test with CH4 + H2O without O2 and that using a bare InGaN.
7. The term “a higher reduction state” may confuse the readers. “a lower oxidation state” would be better.
8. The following sentences need appropriate references.
(Page 2) “It should be noted that that the role of GaN segment in InGaN nanowires is to stabilize the growth of InGaN segments.”
(Page 3) “The room-temperature photoluminescence spectrum shows one strong emission peak at 491 nm and one weak peak at 361 nm (Figure 1f), which are assigned to InGaN segments and GaN segments, respectively.”
(Page 4) “It should be noted that the surface indium (In) site in InGaN is the more active site for methanol production compared to the surface gallium (Ga) site.”
9. The authors explained that the methanol production rate was only slightly reduced when replacing H2O with D2O. I cannot agree with this since the rate was decreased almost by half by the replacement (from 21.4 to 11.0 µmol cm-2 h-1).
10. The authors should describe characteristics of InGaN especially as a photocatalyst in the introduction to make their strategy clearer. Why is InGaN employed in this study?

The answers to the reviewer's comments are sound and sufficient.
The impact and quality of the paper has been improved significantly by the revision.
Thus, the revised manuscript is now acceptable for the publication in Chemical Science.