U.v. photolysis of 13CO enriched pentacarbonyliron in SF6 and Ar matrices at 20 K indicates that tetracarbonyliron has a C2v structure, with bond angles ca. 145 and 120°, as estimated from i.r. intensities. Photolysis of [Fe(CO)5] in an N2 matrix produces [Fe(CO)4] which can be reacted reversibly with the matrix to form [Fe(CO)4(N2)]. Equivalent experiments in CH4 or Xe matrices produce two [Fe(CO)4] species; one is [Fe(CO)4] with the same structure as in other matrices, and there is strong circumstantial evidence for the other being [Fe(CO)4(CH4)] or [Fe(CO)4Xe]. The behaviour of [Os(CO)5] is similar. The results are compared with those obtained in frozen hydrocarbon glasses at 77 K. In matrices with high concentrations of [Fe(CO)5](>1 : 1 000 carbonyl compound : matrix), polynuclear iron carbonyls, [Fe2(CO)8], [Fe2(CO)9], and [Fe3(CO)12], are formed.