Accumulation of products is a known phenomenon associated with a continuously stirred tank reactor (CSTR). Secondary reactions of alpha-olefins due to prolonged bed/pore residence time can significantly change the primary Fischer-Tropsch product distribution. Using D-2 as a tracer, this study first investigated the significance of Fischer-Tropsch product accumulation in a CSTR. Secondly, the D-2 tracer study was used to investigate primary product distribution and olefin to paraffin ratios. Based on the D-2 study, it was found that Fischer-Tropsch synthesis with a 2.5 % Ru/NaY catalyst follows a single alpha mechanism with a chain growth probability of about 0.74. Both olefins and paraffins are primary products and the ruthenium catalyst produced a similar olefin/paraffin ratio for each carbon number. The apparent decline of the O/P ratio for higher carbon number products was shown to be due to secondary reactions of the olefin at prolonged residence times. D-2 tracing was shown to be a versatile tool to investigate product accumulation and to define primary product distribution which is very important for mechanistic interpretation and kinetic modeling.