Abstract The interaction of pyridine and 4-methylpyridine with the square-planar nickel complexes of three salicylaldimine type ligands has been investigated. Spectroscopic studies indicate that the bases add in a stepwise manner such that five-coordinate intermediates are produced; however in each case the final product is six-coordinate. In contrast, addition of excess 2-methylpyridine yields only the corresponding monobase adducts. Combination of solution magnetic susceptibility studies with the spectroscopic data has led to estimation of stepwise stability constants. For each compound K 2 > K 1. Thermometric titrations have also been performed and have enabled calculation of the corresponding ΔH 0 and ΔS 0 values. Attempts to isolate the five-coordinate species have not been successful but a range of bis-base adducts has been isolated and characterised.