A novel in situ device for measuring solubilities of moderately soluble substances such as high-molecular-weight carbohydrates is described. A known sample mass is added to a thermostated cell that is agitated and heated at a steady rate, and the refractive index is monitored until a sudden change in slope is observed, signaling total dissolution of all solids. Two versions of the device are described: one contained about 30 mL of solution, and the second one contained about 3 mL of solution for use with small amounts of solids (30-200 mg). Solubilities measured with both in situ devices are virtually identical and agree closely with literature data; the average deviation from literature values is less than +/-0.2 wt % for both devices. Data, collected at two heating rates (3.9+/-1.7 and 10.2 +/-2.1 degreesC/h), indicate that the fast heating rate gives solubilities statistically identical with those given by the slow heating rate, meaning that reliable data can be collected at the fast rate within 3 h. This new device requires significantly shorter times and less solid consumption than the classical batch sampling approach. It is also better suited for high-temperature measurements than the classical batch sampling approach commonly used to determine carbohydrate solubilities.