The trapping times for photogenerated electrons on 20 Angstrom TiO2 and CdS nanoclusters in acetonitrile are determined from ultrafast transient absorption data. Effective modeling of the data is achieved by employing a kinetic scheme which includes first-order electron trapping and electron-hole recombination kinetics. The fits of the model to the experimental data yield trapping times for TiO2 and CdS of 180 +/- 10 and 90 +/- 20 fs, respectively. The values are slower than are predicted by bulk diffusional or quantum mechanical estimates. A mechanism which allows for slower electron transport via surface trap-to-trap hopping is discussed.