Presented here is a detailed description of a magnetic osmometer as well as the theory of its operation. This instrument has the pressure sensor and is placed on the dialysate side of a semipermeable membrane and is capable of measuring pressure changes as low as 1×10−3cm of water. The stability of the system is greatly enhanced by maintaining contact with a large thermally stable atmosphere. This eliminates ordinary pressure fluctuations encountered when the system is left open to the normal atmosphere. Measurements with bovine serum albumin at concentrations of 86 mg/ml (1.3 mM) were made with an absolute error of ±15% and standard error of ±5% in the pressure difference of 0.0032 cm of water. The molecular weight of this protein was determined to within ±0.4% of the value known from the amino acid sequence. The design presented here facilitates sample changes and requires less than one milliliter of solution per measurement.