A reversed-phase coupled column separation (CCS) system for the analysis of two diastereomeric glucuronides of almokalant, a new class III antiarrhythmic drug, in human urine is described. After direct injection of urine samples (50 microliters) the glucuronides were isolated by complex formation on a terbium(III) loaded strong cation exchanger at alkaline pH. The solutes were eluted from the precolumn by an acidic mobile phase, enriched and separated on Hypercarb (porous graphitic carbon) as analytical column with 0.1 M acetic acid pH 2.8 and 30% acetonitrile as mobile phase. The calibration graph was linear (r2 = 0.9999) and the detection limits were in the low picomole (UV) or femtomole (fluorescence) range. Optimization of the analytical column revealed that elution order and selectivity for the glucuronides were dependent on the buffer agent and temperature used. By appropriate choice of mobile phase conditions all four diastereomers could be separated.