Four 1-oxo-cycloalkan-2-ylideneacetic acids, differing in the size of the aliphatic ring and in the nature of the condensed unsaturated cycle, exhibit different reactivities towards hydrazine, leading in only one case to the desired tryciclic pyridazinone. The observed differences in behaviour cannot be ascribed to different configurations at the exocyclic double bond of the four acids: 1H NMR experiments, combined with UV photolysis, have shown that all compounds have been prepared in the E form and are stable in the absence of light and catalysts. The photochemically obtained Z isomers show an increasing tendency to form tricyclic lactones with increasing size of the aliphatic ring. A theoretical structural analysis of the E and Z isomers, the tricyclic lactones and some of the hypothetical intermediates of the reaction with hydrazine suggests the size of the aliphatic ring and the associated flexibility to be crucial in modulating the ability of these compounds to form tricyclic products.