In this work biodegradation of some Turkish lignite coals by Streptomyces setonii 75 Vi2 in submerged batch cultures was investigated. The effects of coal type, particle size, preoxidation of coal by HNO3, and the starting pH of the microbial culture on solubilization were investigated. It was observed that coal samples that contain the highest amount of lignin solubilize most extensively. The smaller particle size coals solubilize more, and solubilization appears to occur at the external surface of the coal. Under favorable conditions liquid product yields as high as 40% by weight can be achieved.