The interconversion of the two allotropes of the hydrogen molecule (para-H-2 and ortho-H-2) incarcerated inside the fullerene C-60 is reported (oH(2)@C-60, respectively). For conversion, oH(2)@C-60 was adsorbed at the external surface of the zeolite NaY and immersed into liquid oxygen at 77 K. Equilibrium was reached in less than 0.5 h. Rapid removal of oxygen provides a sample of enriched pH(2)@C-60 that is stable for many days in the absence of paramagnetic catalysts (half-life similar to 15 days). Enriched pH(2)@C-60 is nonvolatile and soluble in organic solvents. At room temperature in the presence of a paramagnetic catalyst (dissolved 02 or the nitroxide Tempo) a slow back conversion into oH2@C60 was observed by H-1 NMR. A bimolecular rate constant for conversion of pH2@C60 to oH(2)@C-60 using Tempo of k(Tempo) similar to 4 x 10(-5) M-1 s(-1) was observed, which is approximately 3 orders of magnitudes slower than that for dissolved pH(2) in organic solvents which is not protected by the C-60 shell.