The photochemical interconversion of the two allotropes of the hydrogen molecule [para-H-2 (pH(2)) and ortho-H-2 (oH(2))] incarcerated inside the fullerene C-70 (pH(2)@C-70 and oH(2)@C-70, respectively) is reported. Photo-excitation of H-2@C-70 generates a fullerene triplet state that serves as a spin catalyst for pH(2)/oH(2) conversion. This method provides a means of changing the pH(2)/oH(2) ratio inside C-70 by simply irradiating H-2@C-70 at different temperatures, since the equilibrium ratio is temperature-dependent and the electronic triplet state of the fullerene produced by absorption of the photon serves as an "on-off" spin catalyst. However, under comparable conditions, no photolytic pH(2)/oH(2) interconversion was observed for H-2@C-60, which was rationalized by the significantly shorter triplet lifetime of H-2@C-60 relative to H-2@C-70.