This work is concerned with how gas chromatographic (GC) retention times are determined for adsorption measurements. The standard way is to add a pulse (slug) of one of the pure gases using either an injection valve or a syringe. A chromatographic detector is used to determine the time taken for the pulse to travel through the column. The whole procedure is then repeated using a pulse of the other pure gas. Experience from other workers would suggest that the two retention times are not the same. We take a novel approach. We add pulses of the same size but of different composition. That is, we use mixture pulses. Our investigation involves the ethane-helium-13X zeolite system at 50°C and we consider only one main (carrier) flow. Our results show that there is a linear variation between the composition of the pulse and the retention time. Indeed, it is not sufficient to simply add the retention times for the two pulses and take the average. The standard GC detector is based on thermal conductivity measurements (TCD). In a novel development, we also show how a detector based on the measurement of viscosity can be used to determine these retention times.