The i.r. spectra of [{Ru(π-C₅H₅)(CO)₂}₂], (Ru I), and the new complexes [{Ru(π-MeC₅H₄)(CO)₂}₂], (Ru II), and [{Ru(π-indenyl)(CO)₂}₂], (Ru III), have been investigated between 1700 and 2100 cm.^–1. They are best interpreted in terms of complex equilibria involving cis and trans, bridged (b)- and nonbridged (nb)-tautomers. The b–nb isomer ratio is dependent on substituents on the cyclopentadienyl ring, and variation in solvent, and temperature. The proportion of nb-species increases with increasing temperature, but declines along the series (Ru I) > (Ru II) > (Ru III), and heptane > carbon disulphide > xylene > chloroform > tetrahydrofuran > acetonitrile. There is no evidence for the presence of the nb-isomers in solutions of (Ru III) in any solvent at any temperature. The relative intensities of absorption bands due to either b- or nb-isomers do not vary appreciably with changes in ring substituent or temperature, but are affected by solvent for all three complexes.