The high frequency GMI effect has been investigated on a variety of samples of Fe(62.5)Co(6)Ni(7.5)Zr(6)Cu(1)Nb(2)B(15), in the as-quenched state and submitted to conventional and joule heating thermal treatments, A comparison between high and low frequency GMI measurements is made, along with the study of the variation of the GMI effect as a function of both types of thermal treatments and of frequency, The high frequency low value of the GMI effect can be connected to the frequency dependence of the permeability relaxation, indicating that these soft ferromagnetic alloys are appropriate to be successfully utilised only at low frequencies.