The high symmetry and stability of phenalenyl systems, both as the planar π-radical (P•) and as the π-cation (P⁺), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P• identifies its spontaneous self-association to the diamagnetic P₂, previously characterized as the stacked π-dimer by X-ray crystallography. Likewise, the rapid cross-association of P• with the closed-shell P⁺ leads to the stacked π-dimer cation P₂^•+ with the “doubled” ESR spectrum diagnostic of complete (odd) electron delocalization. These π-associations are confirmed by UV−vis studies that reveal diagnostic near-IR bands of both P₂ and P₂^•+strongly reminiscent of intermolecular charge-transfer absorptions in related aromatic (donor/acceptor) π-associations. Ab initio molecular-orbital calculations for the π-dimer P₂ predict a binding energy of ΔE_D = −11 kcal mol^-1, which is in accord with the experimental enthalpy change of ΔH_D = −9.5 kcal mol^-1 in dichloromethane solution. Most importantly, the calculations reproduce the intermonomer spacings and reveal the delicate interplay of attractive covalent and dispersion forces, balanced against the repulsions between filled orbitals. For comparison, the binding energy in the structurally related cationic π-pimer P₂^•+ is calculated to be significantly larger with ΔE_P ≈ −20 kcal mol^-1 (gas phase), owing to favorable electrostatic interactions not present in the neutral π-dimer (which outweigh the partial loss of covalent interactions). As a result, our theoretical formulation can correctly account for the experimental enthalpy change in solution of ΔH_P = −6.5 kcal mol^-1 by the inclusion of differential ionic solvation in the formation of the π-pimer.