Thermal expansion measurements have been performed on Fe3-xMnxSi alloys in the concentration range 1.2 less than or equal to x less than or equal to 1.8 to study the effect of Mn-substitution on their thermal and magnetic properties. A theoretical model has been used to determine the vibrational and the magnetic contributions to thermal expansion, specific heat and entropy and to obtain the various Gruneisen parameters. (C) 1998 Elsevier Science B.V. All rights reserved.