Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH, thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below -50 degrees C. Tellurocarboxylic acids (5) were observed for the first time by acidolysis of the corresponding cesium tellurocarboxylates with hydrogen chloride. The telluroic acids (6) exist predominantly in the telluroxo form (RCTeOH, telluron acid) in THF at temperatures below -70 degrees C. Telluron acids were reddish to blue violet for the aliphatics (R = alkyl) and dark green for the aromatics (R = aryl) and reacted with aryl isocyanates at -70 degrees C to give crystalline acyl carbamoyl tellurides in good yields.