Metamagnets have been playing an important role as model systems in the physics of phase transitions for 30 years. Especially the isolating substances FeCl2 and FeBr2 have attracted much interest both in experimental and theoretical physics. In this thesis we focus on experimental investigations of Fe1-xMgxBr2 with x = 0 and 0.05. By SQUID-magnetometry, SQUID-susceptometry, elastic neutron scattering and calorimetry we studied the magnetic phase boundaries and especially the recently found new phase transition at H=H1(T) (Aruga Katori et al. 1996). Also we arrived to a physical understanding of the non-critical fluctuations at H=H-(T) in terms of a smeared 2-dimensional phase transition of the spin down sublattice.