An apparatus was constructed to measure directly, to 900 bars, the difference between the isothermal compressibilities of two seawater solutions whose compositions differ with respect to one component. The instrument is being used to determine how specific sea salts affect seawater compressibility; this information will be used to study hydration of ions in seawater. The main design feature of the apparatus is that two equal‐volume bellows‐type densimeters are arranged back‐to‐back inside the same high‐pressure vessel. In operation, one bellows is filled with a test solution and the other bellows is filled with a reference solution. As hydrostatic pressure is applied to the two solutions the bellows compress differentially owing to the difference between the compositions of the reference and test solutions. The differential displacement of the two bellows is measured with a linear differential transformer network, and it is directly related to the difference between the isothermal compressibilities of the test and reference solutions. The apparatus has been calibrated using an absolute technique. In a series of trial runs at 15°C, as a check on the calibration, where the reference solution was distilled water and the test solution was seawater of 35‰, the precision of the method ranged from ±0.03×10−6to ±0.07×10−6bar−1over the pressure range 100–900 bar. The actual compressibility difference values ranged from −3.42×10−6±0.06×10−6bar−1at 100 bar to −2.41×10−6±0.03×10−6bar−1at 900 bar; these values are within +0.01×10−6to −0.39×10−6bar−1, respectively, of one set of present literature values. Some additional seawater measurements made at 10°C were within −0.06×10−6to −0.12×10−6bar−1of the most recently determined compressibility measurements carried out by Bradshaw and Schleicher who used a glass dilatometer. Also some measurements made at 25°C on 0.7615mNaCl vs deionized‐distilled water compared within −0.07×10−6to −0.22×10−6bar−1with the recently reported compressibility data of Milleroet al., who made their measurements using a high‐precision magnetic float densimeter.