Single-well tracer testing has been widely accepted as a standard method for measuring residual oil saturation to waterflood. Residual oil saturation is an important parameter in the evaluation of tertiary oil recovery potential for depleted reservoirs. At an advanced stage of depletion, Leduc, a Canadian carbonate reservoir, has been considered as a candidate for enhanced oil recovery. As part of the evaluation process, single-well tracer tests were conducted at two watered-out producers to determine residual oil saturation to waterflood. The tracer production profiles were found to be highly skewed with long tails and early arrival times, which are typical for carbonate reservoirs. Two different models, namely a double-porosity model where tracer could distribute between the flowing and non-flowing pores through mass transfer and a single-porosity model where a fictitious water drift rate was assumed in the test zone, were used to interpret the data. It was found that either model could match the data to the same degree of accuracy regardless of the flow mechanisms assumed and the residual oil saturation derived from these two models were 35% and 38% respectively. This demonstrates the robust nature of the test that the non-uniqueness of the match does not affect residual oil saturation determination. The residual oil saturation determined by simple analytical models including mass balance method, peak method and mean retention volume method were all in the range of 34% to 38%, in excellent agreement with the simulation results. As well, the Sorw obtained by the SWTT method compared favorably with those determined by interwell tracing (35%) and sponge coring (33%).