The paper describes a method for assigning the C-O stretching bands in cis-disubstituted metal carbonyls of the types L2M(CO)(4), (L-L)M(CO)(4), LL'M(CO)(4) and (L-L')M(CO)(4). The method was based on the parameter delta, defined as delta = (lambda(1)-lambda(2))/(lambda(1)+lambda(2)-2lambda(4)) where lambda(1), lambda(2) and lambda(4) are the lambda parameters of the a(1)((1)), a(1)((2)) and b(2) modes, respectively. For a large number of complexes with the above general formulas, the average value of delta was found to be 0.62, with a standard deviation of 0.02. It was shown that this range of delta can be used as a criterion of the correct band assignment for the complexes understudy. (C) 2002 Elsevier Science B.V. All rights reserved.