Intensity and linewidth measurements of light scattered by the isobutyric acid in water system are reported for various equilibrium states in the critical neighborhood of the temperature-concentration diagram. By extrapolation of these data to zero scattering angle and then to (unrealized) states below the phase separation temperature one may determine a common "pseudospinodal curve" ${T}_{\mathrm{sp}}(X)$ described by ${T}_{\mathrm{c}}\ensuremath{-}{T}_{\mathrm{sp}}(X)\ensuremath{\sim}{|X\ensuremath{-}{X}_{\mathrm{c}}|}^{\frac{1}{{\ensuremath{\beta}}^{\ifmmode\dagger\else\textdagger\fi{}}}}$ where $X$ is the concentration, and we find ${\ensuremath{\beta}}^{\ifmmode\dagger\else\textdagger\fi{}}\ensuremath{\simeq}0.37\ifmmode\pm\else\textpm\fi{}0.04$. As expected on the grounds of the homogeneity hypotheses, the value of ${\ensuremath{\beta}}^{\ifmmode\dagger\else\textdagger\fi{}}$ is essentially the same as the previously observed value of the exponent $\ensuremath{\beta}$ for the coexistence curve. Empirical equations of the form $I_{C,0}^{}{}_{}{}^{\ensuremath{-}1}\ensuremath{\propto}{[T\ensuremath{-}{T}_{\mathrm{sp}}(X)]}^{\ensuremath{\gamma}}$ and $D\ensuremath{\propto}{[T\ensuremath{-}{T}_{\mathrm{sp}}(X)]}^{{\ensuremath{\gamma}}^{*}}$ are used to effect the extrapolations to determine ${T}_{\mathrm{sp}}(X)$. Here ${I}_{C,0}$ and $D$ are the extrapolated zero-angle scattering intensity and the diffusion coefficient, while $\ensuremath{\gamma}$ and ${\ensuremath{\gamma}}^{*}$ are corresponding critical exponents. We show theoretically, however, that a value ${\ensuremath{\beta}}^{\ifmmode\dagger\else\textdagger\fi{}}\ensuremath{\ne}\frac{1}{2}$ is inconsistent with the general validity of these empirical formulas, which should thus be discarded as over-all representations of the variations of $D$ and ${I}_{C,0}$. A tentative test is made of a more general scaling equation for ${I}_{C,0}$ by a convenient plot. Moderate success is obtained. The measurements confirm the exponent values $\ensuremath{\gamma}=1.24\ifmmode\pm\else\textpm\fi{}0.03$ and ${\ensuremath{\gamma}}^{*}=0.67\ifmmode\pm\else\textpm\fi{}0.03$. The distinction between pseudospinodal curves, determined by extrapolation from stable thermodynamics states, and a true spinodal curve which (if it exists) can only be observed by measurements on metastable states, is emphasized.