Pressure-composition isotherms (PCI's) help to determine thermodynamic properties related to hydrogen uptake of materials. PCI's are normally obtained volumetrically with a Sieverts type apparatus or gravimetrically with a microbalance. A potential problem with the gravimetric technique is that the sample is momentarily exposed to air when transferring it to the system often causing unwanted changes such as oxidation and reaction with moisture in the air. In this study, a high-pressure microbalance was built from scratch inside a glove box with inert atmosphere. The system consists of an electromagnetic microbalance, pressure resistant casing for up to 100 bar hydrogen, a flow system for hydrogen and inert gas, heating elements for temperature control, and software for controlling the system. Thermal convection effects are observed and dampened by heating on both the sample and a counterweight. The precision of the mass measurements for a 1 g sample was +/- 5 mu g, and this range proved to be the same independent of pressure and temperature.