A magnetic characterization of the CO
1-xPt
xsystem with

has been made. The anisotropy field (H
A) of both the hard magnetic phases, i.e. hexagonal at low Pt content and tetragonal around the equiatomic composition, has been measured by the singular point detection technique (SPD). The dependence of H
Aon composition and temperature is given. The solid state transformation diagram at low Pt content has been completed by using the thermomagnetic analysis (TMA).