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Infrared polarimetry is a natural progression from infrared photometry, which is becoming a routine astronomical technique. The extra information provided by polarimetric measurements is useful for the investigation of magnetic fields, the composition, alignment and distribution of dust grains, and emission mechanisms. To date, very few infrared polarimetric measurements have been made at wavelengths longer than A new type of polarimeter has been designed and constructed for infrared astronomy. It incorporates a slowly-rotating wire-grid analyser and is completely helium-cooled in order to minimize thermal emission from the analyser. The drive is produced by a magnetic coupling across the dewar wall which eliminates heat leak and maintains vacuum integrity. Spatial chopping at 'v 20 Hz is performed by a helium-cooled self-resonant tuning-fork chopper, mounted at the focal plane in front of a Fabry lens. The entire polarimeter and photometer assembly is situated inside a standard helium dewar which can be mounted at the Cassegrain focus of a telescope. The instrument was designed to be used with the Imperial College 41" balloon-borne telescope for far-infrared astronomy.