The Martensitic Transformation (MT) in Ni–Mn–In–Co metamagnetic shape memory alloys shows isothermal behaviour during the Austenite–Martensite phase change. The time evolution of the magnetization, M (t), under different applied fields and at different temperatures, upon stopping the cooling/heating during the direct/reverse MT has been measured in order to analyze its isothermal characteristics. It has been found that M (t) follows a logarithmic dependence upon isothermal dwellings, both on cooling and heating. The regularities shown by the isothermal transformation rates can be related to a relevant role of the number of interphase boundaries in controlling the isothermal kinetics in this alloy.