The pseudo-gem cinnamophanedicarboxylicacid 1 was shown to undergo a stereospecific [2 + 2] photocycloaddition in the solid state, generating the truxinic acid derivative 2 in 100% chemical yield; the reaction can proceed to completion because it is not an “independent site” solid state reaction. The X-raystructure of 1 reveals that its molecules are associated in dimers formed from four almost