Zinc sulfide microcrystals fully modified with ethanethiol, butanethiol, and octanethiol of about 4 nm diameter with a narrow size distribution were prepared. The rate of photoinduced electron transfer from the microcrystals to methyl viologen (MV(2+)) in solution showed a linear dependence on the concentration of MV(2+), while at bulk ZnS particles the Langmuir-Hinshelwood type kinetics prevailed. It is concluded from the observed logarithmic dependence of the rate constant on the inverse of the chain length of the modified alkanethiols that the electron tunneling controls the photoinduced reduction of MV(2+).