Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (M _s). It has been reported that the presence of martensite formed prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. This kinetic effect is commonly attributed to the creation of potential nucleation sites at martensite-austenite interfaces. The aim of this study is to determine qualitatively and quantitatively the effect of a small volume fraction of martensite on the nucleation kinetics of the subsequent transformation. For this purpose, dilatometry experiments were performed at different temperatures above and below the M _s temperature for athermal martensite in a low-carbon high-silicon steel. Microstructural analysis led to the identification of the isothermal decomposition product formed above and below M _s as bainitic ferrite. The analysis of the transformation processes demonstrated that the initial stage of formation of bainitic ferrite at heat treatments below M _s is at least two orders of magnitude faster than above M _s due to the presence of martensite.