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We study exact solutions of the Einstein-Maxwell equations for the interiorgravitational field of static spherically symmetric charged compact spheres.The spheres are composed of a perfect fluid with a charge distribution thatcreates a static radial electric field. The inertial mass density of the fluidhas the form $\rho (r) = \rho_o + \alpha r^2$ ($\rho_o$ and $\alpha$ areconstants) and the total charge $q(r)$ within a sphere of radius $r$ has theform $q = \beta r^3$ ($\beta$ is a constant). We evaluate the critical valuesof $M/R$ for these spheres as a function of $Q/R$ and compare these values withthose given by the Andr\'{e}asson formula.

Exact solutions for charged spheres and their stability. I. Perfect Fluids