The cyclobutanone and cyclopentanone radical cations were generated in a halocarbon matrix by ionizing radiation at 77 K. The ESR spectrum of a cyclobutanone radical cation consisted of a triple-quintet hf structure with ca. 0.9 mT (2H) and 2.4 mT (4H) splittings. Whereas, both cyclopentanone and 2,2,5,5-tetradeuteriocyclopentanone radical cations gave an identical triplet with 1.5 mT (2H) splitting. Based on the H-1 hf splittings, it was concluded that the SOMO in cycloalkylketone radical cations was changed from a pi(CH2) oribital to a n(0) (and sigma(C-C)) orbital (i.e., pi(CH2)-n(0) orbital crossover) when the ring size increased from a four-member to a five-member.