A procedure by which crystalline sodium norcarnitine can be prepared in large quantities and high yields has been developed. Carnitine is selectively demethylated by thiophenoxide ion in N,N-dimethylethanolamine. The reactive thiophenoxide ion is generated in situ by addition of thiophenol to this basic reaction solvent. Hence, sodium thiophenoxide, which has been used in similar applications, but is difficult to prepare, can be avoided. Accordingly, reaction of (R,S)-carnitine followed by aqueous azeotropic distillation of byproducts as well as excess starting materials and then by neutralization with sodium hydroxide gave sodium norcarnitine in 89% yield. (R)-Carnitine gave 91% yield of (R)-norcarnitine zwitterion before neutralization. A method for the facile preparation of radiolabeled (R)-carnitine is also described. Thus, methylation of sodium norcarnitine with methyl iodide in methanolic acetone produced carnitine, which precipitated, and sodium iodide, which was soluble.