Spectroscopic properties for a series of 2,5-diarylidene-cyclopentanones in weak and strong acid environments are presented.  Electronic absorption and fluorescence spectra have been measured for the all-E configurations of 2,5-dibenzylidene-cyclopentanone (1), 2,5-bis-(3-phenyl-allylidene)-cyclopentanone (2), and  2,5-bis-(5-phenyl-penta-2,4-dienylidene)-cyclopentanone (3) in acetic acid and sulfuric acid solutions.  The spectroscopic evidence indicates that in 96 % sulfuric acid 1, 2, and 3 are protonated both on the ground state and S₁ potential energy surfaces.  This assignment is supported by ZINDO and TD-DFT calculations.  In glacial acetic acid 1, 2, and 3 are unprotonated in the ground state.  The absence of observable fluorescence from 1 in glacial acetic acid indicates that S₁ is nπ*; whereas, the observation of fluorescence from 2 and 3 in acetic acid is consistent with S₁ being ππ*.  A combination of spectroscopic data, molecular orbital calculations, and fluorescence lifetime measurements indicate that 2 and 3 undergo intermolecular excited state proton transfer in glacial acetic acid and diluted sulfuric acid solutions.