The present study for the ketene and methylketene radical cations involves geometry, conformation, and hyperfine structure. The geometry and conformation (for the methylketene cation) investigations were based on the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) calculations, and the isotropic proton hyperfine coupling constants (a(H)) were calculated using various density functional theory (DFT) methods and ab initio UHF, UMP2, and multireference single and double excitation configuration interaction (MRSDCI) methods with large basis sets. The ketene cation is predicted to have a C-2v geometry. The barriers to methyl group rotation in the methylketene cation is predicted to be very low (<1 kcal/mol) and the cis (staggered) conformation is the most stable. On the rotation, the sum of the B3LYP/6-311G(d,p) a(H) values on the three beta protons in the methylketene cation is almost a constant approximately equal to three times the experimental beta coupling value. All the three ab initio methods predict too small a(H) values for the beta protons. The B3LYP (also B3P86 and B3PW91) calculations predict a(H) values for the alpha and beta protons in the two radicals in excellent agreement with experiment and the DFT methods do not underestimate the beta couplings. The UMP2(FULL)/6-311G(d,p) hyperfine structure calculations predict a(H) values for the alpha protons in excellent agreement with experiment and the UMP2 hfcc results are overall better than the MRSDCI results. As an ab initio method for hyperfine structure calculations, the UMP2 method is suggested.