Paramagnetic resonance has been used to follow the process of aggregation of divalent manganese ions present as substitutional impurities in sodium chloride. Samples containing 7 × 10^-5 mole fraction of Mn²⁺ have been aged at temperatures between 20 and 90°C. After a preconditioning heat treatment at 490°C nearly all the Mn²⁺ is present in the form of dipoles (Mn²⁺-vacancy combinations). It is shown that, in the early portions of the subsequent ageing process at least, the ageing behaviour is consistent with the assumption that three dipoles associate to form a relatively stable `trimer', that is, a third-order process is indicated. The activation energy for dipole diffusion is found to be 0·80 ± 0·05 ev. A further aggregation process yielding relatively large aggregates is demonstrated, but the mechanism of this is not clear.