The enzyme 6-oxocamphor hydrolase (OCH) from Rhodococcus sp. NCIMB 9784 catalyses the cleavage of a carbon-carbon bond between two carbonyl groups in both mono- and bicyclic non-enolisable -diketone substrates. In this mode OCH has been shown to effect the desymmetrisation of both bridged symmetrical bicyclic 2.2.1 and 2.2.2 systems and a series of 1-alkylbicyclo3.3.0octane-2,8-diones, yielding chiral substituted cyclopentanone and cyclohexanone products in high optical purity. In the present study, OCH has been challenged with a series of heteroannular substrates including 1-methylbicyclo4.3.0nonane-2,9-dione (7a-methylhexahydroindene-1,7-dione) in an effort to assess the competence of the enzyme for kinetic resolutions of asymmetric, racemic substrates. OCH was shown to catalyse the resolution of 1-methylbicyclo4.3.0nonane-2,9-dione with an E value of 2.9. The effect of increasing the length of the alkyl chain in the 1-position, or enlarging one of the rings, was to increase the enantioselectivity of the enzyme to 5.7 and 3.1 for the substrates 1-allylbicyclo4.3.0nonane-2,9-dione (7a-allylhexahydroindene-1,7-dione) and 1-methylbicyclo5.3.0decane-2,10-dione (8a-methyloctahydroazulene-1,8-dione), respectively. 1-Methylbicyclo5.4.0undecane-2,10-dione (9a-methyloctahydrobenzocycloheptene-1,9-dione) was not a substrate for OCH. These experiments constitute the first description of the resolution behaviour of such a retro-Claisenase enzyme, and suggest a maximum steric limit for substrate recognition by OCH.