Cold cations of small hydrocarbon molecules, i.e., carbocations, are produced in pulsed supersonic molecular beams by a pulsed discharge source. These ions are mass-selected and studied with infrared photodissociation spectroscopy in a time- of-flight mass spectrometer using the method of rare gas atom complex predissociation. Infrared spectra are compared to the predictions of theory (DFT, MP2, CCSD(T)) to elucidate the structures of these ions, their isomers and the potential energy surfaces connecting them. The carbocation species studied include C₂H₃⁺, C₃H₅⁺, C₃H₃⁺, protonated benzene, and protonated naphthalene. Several of these exhibit more than one isomer, allowing investigation of the multiple minima on their potential surfaces. Protonated naphthalene has spectral lines relevant for the three of the main Unassigned Infrared Bands (UIR's) seen in emission from interstellar gas clouds. Oxocations studied with the same methodology include protonated formaldehyde, the acetyl cation, methanol cation and formaldehyde cation.