The spin of pions, a type of elementary particle, was determined by the principle of detailed balance applied to those collisions of protons that produced charged pions and deuterium. p + p → → π π − − + d The known spin values for protons and deuterium allows analysis of the collision cross-section to show that π π − − has spin s π π = 0 . A different approach is needed for neutral pions. In that case the decay reaction into two gamma ray photons with spin one: π π 0 → → 2 γ γ This results supplemented with additional analysis leads to the conclusion that the neutral pion also has spin zero.