Limestone and dolomite are used for retaining sulfur in air-blown gasification systems that are being developed for cleaner coal power generation systems. An intractable deposit was observed to form, apparently from limestone, when a revised base design was being tested in a pilot scale gasifier. The deposit was found to be enriched in Ca and had formed via a melt. The present study has attempted to simulate the formation of melts by sorbent components under laboratory conditions. The detection of melting was accomplished by using impedance spectrometry. This technique measures the change in resistivity when a nonconducting solid forms an ionic melt. The validity of the technique was demonstrated by reproducing melt formation that had previously been observed in the CaO/CO2/steam system under the conditions of the CO2-Acceptor Process. SEM/EDX and XRD were used to characterize the residues from the present tests. CaCO3/CaSO4 was the only system tested that formed a melt under the experimental conditions corresponding to the pilot-plant trials. Melt formation was observed at 1010 degreesC at pressures in excess of 13 bar(a). The melting temperature was not affected by the presence of other Ca compounds (viz. CaS) that would be present in the gasifier. It is thought that CaSO4 formation occurred when CaS in the recirculating sorbent was exposed to the oxidizing conditions of the spout of the gasifier. The melt would have formed by the interaction of CaSO4 with the uncalcined CaCO3 of the sorbent particles. Oxidizing sulfided limestone and dolomite and raising the temperature to above 1010 degreesC, which caused a melt to form, demonstrated the sequence of reactions. The properties of the melts produced from limestone and dolomite were different; these results help to explain why dolomite did not give rise to deposits during pilot plant trials.