Crystal-field infrared active excitations and photoluminescence of Nd 3 + ions in weakly doped LiYF 4 have confirmed that the concentration dependent satellite lines accompanying the Nd 3 + crystal-field optical transitions are due to four ferromagnetically coupled pairs of Nd 3 + ions in undistorted Y 3 + sites with the exchange energies J 1 =0.9, J 2 =1.6, J 3 =3.1 and J 4 =4.5cm - 1 , respectively. A linear Zeeman splitting of the Nd 3 + ion 4 F 3 / 2 -> 4 I 9 / 2 transition is observed and the g-factors (g =0.2+/-0.1; g =0.97+/-0.01) associated with the 4 F 3 / 2 lowest level are determined.