The gas-phase basicity and proton affinity of pentacene have been determined by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. AM1 semiempirical calculations for neutral and protonated linear polyacenes ranging from benzene to pentaicosacene were carried out. The influence of successive annellations (up to 25 rings) on structures, heats of formation, and proton affinities was investigated. Extrapolated values for an infinite linear polyacene have been derived. AM1-calculated electronic structures (HOMO and LUMO energies) for the studied linear polyacenes have been compared with the available experimental ionization potentials and electron affinities.