The energy dispersion of the singlet-triplet dimer excitation in Cs3Cr2CI9h as been studied by inelastic neutron scattering (INS) at temperatures down to 1.3 K. The results can be accounted for by using a completely isotropic Heisenberg Hamiltonian in the random phase approximation (RPA). Only nearest-neighbor interactions need to be considered. From the excellent fit, three exchange parameters were obtained: J = -1.75 meV (intradimer), J = -0.032 meV (interdimer, intrasublattice) and J, = -0.031 meV (interdimer,i ntersublattice). These values indicate that Cs3Cr26I9is far from ordering magnetically. The systematic variation of the exchange parameters on substitution of C1 by Br and I is discussed.