The alloys of Zn0.34Cd0.66S QDs show a high photocatalytic activity when mediating the hydrogen production from water with a reaction rate of 233 +/- 9 mu mol h(-1) g(-1). The reaction is carried out in the presence of ascorbic acid as sacrificial electron donor. The alloy Zn0.34Cd0.66S/ZnS QDs with a ZnS shell showed a yield about seven times lower for the same process compared to the ZnCdS/ZnS alloy. The dynamics of the exciton reactions in the initial stages of the reaction mediated by these QDs was studied by femtosecond laser spectroscopy. Femtosecond transient absorption spectra is reported in a detailed and comprehensive way describing dynamics of the absorption peak A1 near the bleach band of edge exciton B1. This A1 absorption peak is attributed to the Stark shift due to exciton-exciton interactions due to the trapping of a hole or an electron on the QDs surface. In shell-less ZnCdSQDs, the A1 peak presented two kinetic components: the damping of the A1 peak amplitude due to the relaxation of "hot" excitons and the concomitant growth of the A1 peak at later delay times. We suggest that this was due to due to the carrier capture charge by surface traps leading to electric field redistribution in the QDs. The relaxation/decay of "hot" electrons in alloys of ZnCdS and ZnCdS/ZnS QDs occur with characteristic times of tau(ZnCdS) = 153 fs for ZnCdS and tau(ZnCdS/ZnS) = 193 fs.