The magnetic and magnetoelastic properties of YbPO4 and YbVO4 crystals are investigated experimentally and theoretically; the crystal field parameters are determined, as well as the magnetoelastic coefficients Bμ and total quadrupole coupling constants Gμ for all symmetry modes. It is found that, for H ∥ [100], γ-symmetric quadrupole interactions predominate and are responsible for a significant contribution to the third-order susceptibility, magnetization, magnetostriction, and elastic constant. It is demonstrated that, in the absence of an external field, these interactions do not lead to quadrupole ordering, because the respective deformation susceptibility χγ is several times less than the critical value of 1/Gγ. The influence of an external magnetic field along different symmetry axes on the quadrupole effects and quadrupole interactions in Yb zircons is investigated. It is demonstrated that, for H ∥ [110], the susceptibility χγ increases with the field, so that in a fairly strong field in the investigated crystals one can expect a γ-symmetric stimulated phase transition.