[en] A previous method for determination of the radiochemical purity (RCP) value of [¹²⁵I]sodium iothalamate uses two paper strips and solvents (total developing time is ∼2.5 h). To simplify and shorten the RCP testing procedure, our laboratory has developed a single-strip chromatography method that not only distributes free ¹²⁵I and [¹²⁵I]sodium iothalamate to different relative front (R_f) locations, but is also faster and easier to perform. RCP of [¹²⁵I]sodium iothalamate was determined with the use of a 10-cm instant thin-layer chromatography strip impregnated with polysilicic acid gel (ITLC-SA) as the solid phase, and a mobile phase of 2-butanol:acetic acid:water (140:2.5:70, v/v). By using autoradiography and counting the strip segments in a gamma counter, our results indicated that free ¹²⁵I migrated to R_f = 0.89-1.00 while the [¹²⁵I]sodium iothalamate moved to R_f 0.44-0.67. The total developing time for the single-strip ITLC-SA system was ∼ 1 h