A study of the antiferromagnet CeRh2Si2 by torque, magnetostriction, and transport in pulsed magnetic fields up to 50 T and by thermal expansion in static fields up to 13 T is presented. The magnetic field temperature phase diagram of CeRh2Si2, where the magnetic field is applied along the easy axis c, is deduced from these measurements. The second-order phase transition temperature T-N and the first-order phase transition temperature T-1,T-2 (=36 K and 26 K at zero field, respectively) decrease with increasing field. The field-induced antiferromagnetic-to-paramagnetic borderline H-c, which equals 26 T at 1.5 K, goes from first order at low temperature to second order at high temperature. The magnetic field temperature phase diagram is found to be composed of (at least) three different antiferromagnetic phases. These are separated by the first-order lines H-1,H-2, corresponding to T-1,T-2 at H=0, and H-2,H-3, which equals 25.5 T at 1.5 K. A maximum of the T-2-coefficient A of the resistivity is observed at the onset of the high-field polarized regime, which is interpreted as the signature of an enhanced effective mass at the field-induced quantum instability. The magnetic field dependence of the A coefficient in CeRh2Si2 is compared with its pressure dependence, and also with the field dependence of A in the prototypal heavy-fermion system CeRu2Si2.