We have studied the nature and surface morphological changes associated with the photodegradation of stearic acid LB films on TiO2(110). Interestingly, submonolayers of stearic acid consisted of circular domains of various sizes-a feature very attractive for monitoring TiO2 photocatalysis by AFM. We noted that there was no bulk differential reactivity at island edges compared to the interior. This suggests that the rate of photodegradation of the stearic acid molecules is independent of their location in the island. Accordingly, the overall surface reactivity trends were similar for both partial films and complete films. Likewise, the observed inhomogeneous reactivity patterns appear to be a reflection of the transient distribution of the reaction centers.