Polyimide (PI) is an insulating polymer that is finding increased use in the semiconductor industry as an intermetal dielectric, passivation coating, and planarization layer. It has a low dielectric constant (3.5 at audio frequencies) and can be processed at temperatures below 450°C. These features make PI attractive to the integrated circuit industry. One disadvantage of this material is that, like most polymers, it is hygroscopic. Moisture can lead to such reliability problems in integrated circuits as increased insulator conductivity, loss of adhesion, and corrosion. It is important to quantify the moisture uptake in polyimide so that the reliability implications of its use can be fully understood. In this study, the steady-state moisture uptake of the model PI pyromellitic dianhydride-oxydianiline (PMDA-ODA) is reported. The moisture uptake is measured using a Cahn 1000 Microbalance with a 1-μg resolution. The samples are prepared by spin-coating one or more layers of the precursor polyamic acid onto 2-in silicon substrates. The moisture uptake in these polymers is found to be linearly related to ambient relative humidity, and the maximum moisture uptake by weight is 3.2%. The moisture uptake in PI is also shown to be a bulk absorption rather than a surface adsorption phenomenon.