A capacitance relative humidity (RH) sensor is described that has a design, construction, and material composition that result in an inexpensive and robust sensor. This sensor has a multilayer, free-standing film construction. It consists of a humidity sensitive polyimide (PI) dielectric core and conductive layers consisting of carbon filled polysulfone on each side of the polyirnide film to form a capacitor. The polyimide used is a BPDA-ODA type, and replaces a PMDA-ODA type polyimide used in a previous version of this sensor. The BPDA-ODA sensor has a nominal capacitance of 200 pF and a nominal sensitivity of 13% at 100% RH. The characteristics of this humidity sensor are discussed and compared to the characteristics of the PMDA-ODA type sensor. Characteristics considered include the PI film moisture uptake and water vapor transmission, and the sensors' sensitivity to relative humidity, frequency response, and aging at 85°C/85% RH. The dual-state sorption model and free volume calculations are used to demonstrate that observed differences in the film are due to differences in chemical composition between the films.