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Measuring the temperature dependence of the ac-initial susceptibility chi-ac (T) of Nd2Fe14B, two peaks were found. One peak, which is at 140 K, corresponds to the occurrence of the spin reorientation transition. The second one, which is at 220 K, can be correlated to the temperature where the FOMP (first order magnetization process) transition starts. A similar behaviour was found in Nd2Fe14C and Nd2Co14B also. Only one peak was detected for Pr2Fe14B and Pr2Co14B, suggesting that no spin reorientation but a FOMP does exist in these compounds. Direct measurements of the temperature dependence of the critical FOMP field and the anisotropy field using the SPD technique prove that this peak occurs due to the FOMP transition. A theoretical, however phenomenologically based, analysis gives an explanation for the occurrence of these peaks in chi-ac (T). Furthermore, it can be shown that the peak in the ac-initial susceptibility, which indicates the onset temperature of the FOMP, can be detected only by measuring either on polycrystalline samples or applying the external ac-field in some special crystallographic directions of a single crystalline sample. The peak which corresponds to the occurrence of a FOMP will disappear if the external ac-field is applied parallel to the c-axis or preferential axis of a magnetically oriented sample. This gives an opportunity to differentiate a peak due to a FOMP from that of a spin reorientation by ac-initial susceptibility measurements. It should be mentioned that this new and simple method gives the possibility to get direct information whether there exists a FOMP transition in a compound or not. This is especially important if a FOMP occurs at very high fields which are far beyond the limit of the current facilities. This allows also the experimental test of CEF calculations which predict sometimes a FOMP transition in high magnetic fields and the critical temperature range where high order CEF terms can or cannot be neglected.