The FTIR spectra in solution and vapour phase of the compounds, cyclopentadienyl manganese tricarbonyl (h-C5H5)Mn(CO)3 and methylcyclopentadienyl manganese tricarbonyl (h-C5H4CH3)Mn(CO)3 were measured in the region 4000-400 cm-1. An assignment is offered in which the normal modes are described in term of “Local symmetry”. This assignment has been discussed in term of previous assignment given for (in liquid phase and nujoll mull) other h–cyclopentadienyl and related compounds including ferrocene, dimethyl ferrocene, benzenechromium tricarbonyl and toluene chromium tricarbonyl.