Photoinduced electron transfer from N-alkylphenothiazines (PCn, n = 3, 9, 12, 16) to D2O at a micellar interface was monitored versus the alkyl chain length of PCn and versus cyclodextrin (CD) addition into the micelle by determining the photoproduced radical yield by electron spin resonance. The photoyield of PCn in sodium dodecyl sulfate micelles monotonically decreases with increasing concentration of alpha-CD. This results from an increased electron transfer distance between the phenothiazine moiety and interface D2O by replacing some interface D2O with alpha-CD. Addition of beta-CD shows a different trend for the photoyield. The photoyield increases with increasing concentration of beta-CD up to 10 mM and thereafter monotonically decreases. The increased photoyields up to 10 mM beta-CD result from a decreased interaction distance between the phenothiazine moeity and D2O at the micellar interface by intercalation of beta-CD between the headgroups of the surfactant molecules. The decrease of photoyield above 10 mM beta-CD results from an initial increased interaction distance between the phenothiazine moiety and interface water by replacing some D2O at the micellar interface to 15 mM beta-CD and thereafter disrupting the micellar structure by the high concentration of beta-CD.