Specular Cd1-yMnyTe epilayers of (100) and (111)B orientation have been grown on 5 cm GaAs (100) and GaAs/Si (100) substrates using dimethyl cadmium, diethyl tellurium and the new organomanganese sources RMn(CO)5 (R = C6H5CH2 or CH3). The manganese compounds are volatile, air and water stable solids which are prepared in high yield and are readily purified by sublimation. Growth was performed at 320-350°C in a horizontal reactor at atmospheric pressure. The feasibility of low temperature interdiffused multilayer process (IMP) MOCVD of the quaternary alloy Hg1-x-yMnyCdxTe is demonstrated. Cati on interdiffusion profiles, including the first reported for Mn in II-VI compounds, are obtained by analytical electron microscopy of ultramicrotomed cross sectional samples using a fine 10 nm electron beam probe. Interdiffusion of Mn is comparable to that of Cd. The Cd1-yMnyTe (111)B layers exhibit fine scale 180°C rotational twinning. Growth of untwinned HgTe on Cd1-yMnyTe (111)B buffer layers is observed, with the HgTe layer adopting the orientation of the surface of the buffer layer. Interdiffusion during growth and subsequent annealing yields untwinned (111)B quaternary alloy on the HgTe side of the interface. The structural quality of (111)B and (100) epilayers is compared by lattice imaging and selected area electron diffraction patterns. No voids or precipitates are observed, in contrast to material grown by techniques employing higher temperatures.