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A spaceborne inflatable antenna concept is evaluated for passive microwave sensinlkg of soil moisture and ocean salinity. The concept makes use of a large-diameter, offset-fed, parabolic-torus antenna with multiple feeds in a conical pushbroom configuration. An inflatable structure provides the means for deploying the large-aperture, low-mass, and low-cost antenna system in space, suitable for operation in the 1-3-GHz frequency range needed for soil moisture and salinity sensing. The concept is designed to provide multichannel, constant-incidence-angle, wide-swath, and high-radiometric-precision observations of the earth's surface. These capabilities facilitate estimation of soil moisture and salinity, with global coverage every two to three days. Simulations show that a 25-m diameter, 1.41- and 2.69-GHz, dual-polarized system should be capable of providing surface soil moisture estimates with an accuracy of [approximately]0.04 g-[cm.sup.-3] (where vegetation water content is less than [approximately]5 kg-[m.sup.-2]) at a spatial resolution of [approximately]30 km. Although inflatable systems represent a new and untested technology for remote-sensing applications, the advantages of low packaged volume, low manufacturing cost, and low mass provide an incentive for their study. This paper evaluates one possible concept for incorporating the capabilities of inflatable systems into a scientific mission and for demonstrating these capabilities for remote-sensing applications. Index Terms - Aperture antennas, hydrology, inflatable antennas, microwave radiometry, ocean salinity or sea surface salinity, remote sensing, soil moisture.