This paper focuses on the thermochemistry of some derivatives of cyclopenta-1,3-diene, namely, 5-methylcyclopenta-1,3-diene, 5-ethylcyclopenta-1,3-diene, 5-formylcyclopenta-1,3-diene, 5-methylcyclopenta-1,3-diene-1-yl radical, 5-ethylcyclopenta-1,3-diene-1-yl radical, 5-carbonylcyclopenta-1,3-diene radical, 1-formylcyclopenta-2,4-diene-1-yl radical, 5-methylenecyclopenta-1,3-diene radical, 5-ethylidenecyclopenta-1,3-diene radical, and 3,6-dimethylenecyclohexa-1,4-diene. Several different chemistries of these compounds are of interest in combustion modeling. Here, we present gas-phase thermochemical properties for the above cited species, which are, except for 3,6-dimethylenecyclohexa-1,4-diene, previously unknown. These were obtained from corrected (using bond additivity corrections) high-level ab initio quantum chemistry calculations validated with well-known compounds including cyclopentane, cyclopentene, cyclopenta-1,3-diene, and cyclopentadienyl radical. Heat capacities and entropies have been corrected for anharmonic molecular motions, in particular for internal rotations. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 453–463, 2003