The ferromagnetism of Co/Cu(001) at the transition from islands to closed layer growth is studied by temperature-dependent magnetization and susceptibility measurements in situ in UHV. The Curie temperatures above and below a critical coverage d c ~1.8ML for percolation were measured. Near this particular thickness a discontinuous jump of the Curie temperature of more than 100K is observed. At d c the Curie temperature is metastable and varies strongly with time and heat treatment. Below d c the sample remains ferromagnetic. We find a remanent magnetization and finite coercive fields, which clearly excludes the possibility of superparamagnetic behaviour.