The exchange of the phosphine ligand on the half-sandwich 15-electron, spin quartet [CrCpCl₂L] system has been investigated experimentally by stopped-flow kinetics with visible detection and theoretically by calculations with DFT methods on the PH₃ self-exchange model system. The exchange of PMePh₂ with PMe₃ follows clean second- order kinetics with the activation parameters ΔH ^‡ = 7.0(2) kcal mol^–1 and ΔS ^‡ = –24.3(8) cal K^–1 mol^–1, consistent with an associative exchange. The rate constant for the exchange of L with PMe₃ in [CrCpCl₂L] at room temperature varies only within a factor of 8 for the series of complexes with L = PPh₃, PMePh₂, PMe₂Ph, PEt₃, or η¹-dppe. The computational work showed that the PH₃ self-exchange process occurs via a symmetric transition state along the spin quartet hypersurface, without crossover to the spin doublet state. The optimized transition state corresponds to a first-order saddle point with Cr–P distances of 3.190 and 3.174 Å, located 7.6 kcal mol^–1 above the [CrCpCl₂(PH₃)] (spin quartet) + PH₃ combination, or 13.6 kcal mol^–1 below the [CrCpCl₂(PH₃)₂] doublet minimum. Thus, the phosphine exchange reaction can be classified as a classical S_N2 process.