We have observed anomalously rapid relaxation of dipole-dipole energy in mixed-state superconducting vanadium. We show that spin diffusion driven by an inhomogeneous magnetic field uses spin-spin energy as a heat sink. Coupled differential equations are proposed for magnetization and spin-spin energy, and the diffusion coefficient is inferred from our measurements. It is concluded that spin diffusion cannot be a relaxation mechanism for magnetization in the mixed state, as previously assumed. © 1973 The American Physical Society.