Abstract Enhancement factors of the 119Sn-CIDNP signals observed in hexamethylditin — the recombination product of trimethyltin radicals — are determined. The enhanced absorption in the main 119Sn signal, which cannot be explained by Kaptein's rules based on a first order perturbation treatment of the radical pair theory, can be described by applying the high field approximation of the radical pair model if second order perturbation theory is used. The A/E type multiplet effect in the satellites arising from coupling with 117Sn nuclei which is in accordance with Kaptein's rules is not affected by the second order treatment. Pedersen's analytical expressions derived from the three-dimensional diffusion model describe the 119Sn-CIDNP effects using p = 0.32 and Λ = 1 or p = 0.5 and Λ = 0.56. During the photochemical decomposition of hexamethylditin and octamethyltritin, the main 119Sn signals show enhanced absorption. Enhancement factors found experimentally are compared with values calculated using the second order treatment. It follows that both compounds mainly decompose from triplet states.