Muonium atoms were observed to undergo a fast reaction with several chromium(III) complexes in water, with rate constants in the neighborhood of 10(10) M(-1) s(-1). The reaction occurring is attributed to a simple electron-spin-exchange in muonium, catalyzed by the paramagnetic d(3) metal ions. There is a quantum mechanical statistical factor for this process of 62% per collision, which becomes 50% when multicollisional cage effects in solution are included, with the result that the observed rates are close to the diffusion-controlled limit for a muonium reaction in water. The implication is that nearly half of the encounters between muonium and a Cr complex result in electron spin exchange. These muonium rates are faster by a factor of similar to 5 than the corresponding reactions of positronium. They also show a different dependence on the nature of the ligands. These differences are attributed to positronium being larger and more polarizable than muonium, so that spin-spin coupling with the metal’s d electrons is weaker and more sensitive to the nephelauxetic effect of the ligands in the case of positronium. The corresponding reactions involving ordinary H atoms are not available for comparison but presumably resemble muonium’s.