Infrared argon vibrational predissociation spectroscopy is used to probe the structures of carbocations (C_nH_m)⁺, formed in a pulsed electric discharge through a dilute hydrocarbon supersonic expansion. Size specific infrared spectra are obtained by mass selection in a reflectron time of flight mass spectrometer followed by the resonant photo-fragmentation of the ion with tunable (2-16 μm) output from a pulsed infrared OPO. Spectra are presented for the gas phase t-butyl, protonated benzene, and propargyl cations. A family of cations based on the allyl cation core (C₃H₅)⁺ is formed from several hydrocarbon precursors and appears to be intrinsically stable. Two isomers of (C₃H₅)⁺ (the allyl and propenyl cations) are observed and assigned based on distinct spectral features in the fingerprint region of the infrared spectrum, corresponding to the C-C-C asymmetric stretches.