Polyether polyurethane networks were prepared in solution at 25 and 60degreesC, using various organotin catalysts. The presence of a free-radical initiator [viz. azobisisobutyronitrile (AIBN)] in the reaction media has no effect on the catalytic efficiency of organotin(IV) compounds, whereas it causes severe deactivation of organotin(II) compounds. This effect was explained by the following mechanism: formation of a cyclic 1 : 1 complex by coordination of the nitrile groups of AIBN with the tin(II) atom, which both reduces the effective catalyst concentration and allows the early decomposition into radicals, as previously seen, leading to tin oxidation. The addition of a free-radical scavenger such as 3-tert-butyl pyrocatechol allows the maintenance of the catalyst efficiency of organotin(II) compounds at their reference level.