We analyze particle segregation in suspensions of rigid particles immersed in Newtonian liquids flowing in plane Couette devices by considering the stability of the system to two-dimensional perturbations in the plane of shear. We obtain the dispersion relation analytically for small wave numbers, finding that when the velocity of the walls is kept constant, the system is always stable and the particle concentration remains uniform. On the other hand, when we keep constant the shear applied at one wall, we predict that a disorder-order transition will occur, provided that sigma > 1, where sigma is the non-dimensional parameter, sigma = (mu (0)'(E) over cap (0))/(mu (0)(D) over cap (0)). Here mu (0) and mu (0)' are the effective viscosities of the suspension, mu, and its derivative, d mu /d phi, evaluated at the mean particle volume fraction phi (0), while (D) over cap (0) and (E) over cap (0) are two transport coefficients appearing in the constitutive relation for the material flux. Substituting experimental correlations for (D) over cap (0) and (E) over cap (0) into this condition, we conclude that for no values of phi (0) will particles cluster along the plane of shear.