The use of tetramethylrhodamine isothiocyanate isomers R and G in microbiological and immunochemical procedures has prompted interest in their chemical and physical properties. Tetramethylrhodamine isothiocyanate (TMR) isomer R has been found to be more efficacious than isomer G in immunochemical work (1). Hence, the establishment of the structures of the respective isomers of tetramethylrhodamine isothiocyanates (Figure II) would be of value for better understanding the fluorescent staining and immunochemical activities of these reagents.