An equation for the conservation of density ratio on isopycnal surfaces is derived. It is shown that a vertical shear can modify the density ratio if salinity (and temperature) gradients exist along isopycnals. The mixing processes of turbulence, salt fingering and lateral isopycnal mixing are evaluated for possible balances with the shear term in the “Rp, = constant” Central Waters. It is found that turbulence cannot provide a balance and that isopycnal mixing can balance only with a particular vertical variation in the mixing rate. Salt firm however, provide a simple balance between the shear term and the vertical variation of flux divergence, since beat and salt are transported at different rates. The “conservation of density ratio” should provide a useful constraint on mixing estimates from hydrographic data in both double-diffusive and non-double-diffusive regimes and could be applicable to modelling studies of the temperature-salinity relation in the thermocline.