The effect of donor ligands on the chromium carbonyl photocatalysed hydrosilylation of 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, trans-1,3-pentadiene with triethylsilane was studied. Photocatalytic hydrosilylation of dienes conducted in the presence of Cr(CO)(5)L (L = P(CH3)(3) or P(OCH3)(3)) yields the cis-1,4-adducts, 1-triethylsilyl-2-butene derivatives, as the main products; these have been isolated by distillation or preparative GC and fully characterised by NMR spectroscopy. As with the photocatalytic hydrosilylation of dienes in the presence of Cr(CO)(6), we propose a mechanism which involves the initial conversion of Cr(CO)(5)L into fac-Cr(CO)(3)L(eta(4)-1,3-diene) followed by a further photolytic CO substitution by triethylsilane, forming a Cr(CO)(2)(H)(SiEt3)L(eta(4)-1,3-diene)intermediate. The addition of hydride to diene occurs reversibly to form an eta(3)-enyl ligand prior to the irreversible silyl transfer to the organic moiety. The 1,4-hydrosilylation adduct is then replaced by new substrates to complete the catalytic cycle. Cr(CO)SL complexes form active species which are much more stable than the one generated from CI(CO), in the absence of any donor ligands. Thus, introduction of an equimolar amount of donor ligand (trimethylphosphine or trimethylphosphite) increases the Life of the catalyst. This leads to the achievement of larger turn-over numbers (80 or 70) compared with the value of 30 for just Cr(CO)(6). Cr(CO)(5)L (L = P(C6H5)(3), P(C6H11)(3) or C5H5N) proved inactive in the attempted photocatalytic hydrosilylation of dienes.