<p>The photooxidation of isoprene contributes significantly to the peroxy radical (RO<sub>2</sub>) pool in the atmosphere. With a widespread decreasing trend of nitrogen oxides (NO<sub>x</sub>) emissions, the cross-reactions of isoprene-derived RO<sub>2</sub> with hydroperoxy radicals (HO<sub>2</sub>) become increasingly important. Yet large uncertainties remain in the effect of water vapor on the products yields in these reactions. In the present study, we investigated the photooxidation of isoprene under 30 % relative humidity and 80 % RH in which the cross-reactions of RO<sub>2</sub> and HO<sub>2</sub> were dominated. The experiments were conducted with ozone photolysis as the hydroxy radical (OH) source. We found that in the first-generation reactions, the branching ratios for methacrolein (MACR) and methyl vinyl ketone (MVK) in the cross-reactions of β-isoprene hydroxy peroxy radicals (β-ISOPOO) and HO<sub>2</sub> under 30 % RH and 80 % RH increased to approximately three times and five times of those under dry conditions owing to a water-induced change in the complexation patterns of β-ISOPOO and HO<sub>2</sub>. Based on the branching ratios achieved in this study, we estimated that the MACR and MVK emissions are enhanced by 4.7−12 and 18−34 Tg yr<sup>−1</sup> while the β-isoprene hydroxy hydroperoxide (β-ISOPOOH) emission is suppressed by 39−78 Tg yr<sup>−1</sup> on a global scale when considering the water effect. In the multi-generation reactions, the yields of formic acid (FA) and acetic acid (AA) with water vapor were raised by over fivefold than we expected, able to narrow the bias between the modeled and observed global FA productions by 20 %. Since β-ISOPOOH and MACR, as well as FA and AA, play pivotal roles in aerosol formation and growth, a better interpretation of their yields helps understand the fate of isoprene in the atmosphere and improve the effect of the simulations of isoprene-derived aerosol burdens and chemical compositions.</p>