The photochemical reactivities of 1,1-diarylspiropentanes 1a–c have been investigated under various photolysis conditions. Upon direct photolysis or acetone-sensitized photolysis, 1 underwent skeletal rearrangement to afford methylidenecyclobutanes 2, 3 and 4. A mechanism involving diradical intermediates has been proposed. Also studied were the photochemical and thermal reactions of the electron donor–acceptor (EDA) complexes of 1 with tetracyanoethylene (TCNE). Photoirradiation of the charge-transfer (CT) absorption bands of the EDA complexes resulted in the skeletal rearrangement of 1 to 2 and 3, with concomitant formation of TCNE adducts 11b,c and 12a–c. The X-ray structure of 11c is reported. A mechanism involving ion radical pairs [1˙⁺, TCNE˙^–] is proposed for the photoreaction. In contrast, no skeletal rearrangement was observed in the thermal reaction of the EDA complexes although 11b and 11c were produced.