The magnetic circular dichroism (m.c.d.) spectrum of a single crystal of cubic [Ni(H2O)6](BrO3)2 has been measured at 4 K, and at six temperatures between 77 and 300 K. The m.c.d. parameters for each spin–orbit component of the vibronic d–d bands of the nickel(II) hexaquo-ion have been calculated. The m.c.d. spectrum at 4 K is dominated by C terms arising through both first- and second-order spin–orbit coupling. The signs of the C terms confirm that the dominant active vibration is of t1u symmetry. The quantitative values of C/D, where D is the dipole strength, are determined from a 1/T plot for the ‘red’ band. A comparison with calculated parameters supports the view that the 1aE state is interacting with the 3T1g state leading to the characteristic double-peaked ‘red’ band of the [Ni(H2O)6]2+ spectrum. A pronounced vibrational progression is found in the ‘red’ band at 4 K. The frequencies in the progression can be understood in terms of an interaction between the E spin–orbit components of the 3T1g state and the 1aE(t2g6eg2) state.