The scaling behaviour of the thickness evolution of ferromagnetism in single crystal Fe films on GaAs(100)-4 x 2 and InAs(100)-4 x 2, grown by molecular beam epitaxy, have been studied. In both systems there is a superparamagnetic-ferromagnetic transition whose onset occurs around 4.5 monolayers (ML) Fe. The susceptibility around the transition follows a scaling behaviour; chi similar to (1 - d/d(c))(-gamma) with gamma = 2.13 +/- 0.30 for Fe/GaAs and gamma = 2.21 +/- 0.25 for Fe/InAs, suggesting the 2D percolation of clusters.