[en] This report describes the fabrication and standardisation of an apparatus called, 'Osmistor' for molecular weight determinations. The set up uses a matched pair of thermistors as temperature sensing device and is based on the principles of relative lowering of vapour pressure of a solution with respect to the solvent exposed to the same temperature. The instrument was calibrated using standard solution (0.01 to 0.08 M) of A.R. benzil in three different solvents, viz, benzene, acetone and chloroform at a temperature of 38.0, 25.0 and 25.0 +-0.01deg C respectively and the calibration constant K was determined. The molecular weight of naphthalene in the above solvents was determined with a precision and accuracy of +- 3%. Also, the molecular weights of a few synergistic uranyl complexes were determined with an accuracy of about 5%. (author)