On three fluorides CeF3, NdF3, and PrF3 accurate susceptibilities measurements are reported from 2 up to 300 K under a magnetic field applied along the c axis. On the same samples, the optical Faraday rotation has been investigated in the temperature range 8–300 K at 0.6328 μm wavelength in a magnetic field up to 20 kOe. The reciprocal susceptibilities χ−1(T) follow a Curie Weiss law for T>100 K; the observed values of the paramagnetic Curie temperature are − 56 K (PrF3), − 114 K (CeF3), and − 26 K (NdF3). For T<100 K, strong crystalline field effects are evidenced: for example, a maximum of χ near 60 K has been found in PrF3. In the three fluorides negative Verdet constants have been obtained. The ratio V/χ appears as strongly temperature dependent, that is quite different from the theoretical prediction. Furthermore, the experimental results show an evidence of two behaviors of V/χ for T>77 K. (V and χ are expressed in deg cm−1 Oe−1 and in μB mol−1 Oe−1, respectively.) On one hand for NdF3 and PrF3, it has been found V/χ =A(1+αT) where A is a constant (−4320 in NdF3; −7241 in PrF3); in both samples, α is in the order of −6×10−4 K−1, note that Faraday rotation measurements performed at 0.4579 and at 0.5017 μm lead to an identical α value. On the other hand in CeF3, V/χ is a linear function of T−1 and is given by 13700(1+75.9/T).