  $^{151}$Eu M\"ossbauer spectroscopy shows that yttrium substitution in
mixed-valent $\rm EuPd_3S_4$ drives the initial 50:50 mix of Eu$^{3+}$ and
Eu$^{2+}$ towards pure Eu$^{2+}$, whereas lanthanum substitution has the
opposite effect, but only for substitution levels above 50\%. We find that
total valence electron count and chemical pressure effects cannot account for
the observed behaviour, however conserving the cell volume provides a
consistent description of the changes in the Eu$^{2+}$:Eu$^{3+}$ ratio.
Remarkably, lanthanum substitution also leads to a clear transition from static
mixed-valent behavior at lower temperatures to dynamic mixed valent behavior at
higher temperatures, with the onset temperature monotonically increasing with
Eu content and extrapolating to a value of $\sim$340~K for the pure $\rm
EuPd_3S_4$ compound. Magnetic order persists at least as far as x=0.875 in both
series, despite the drastic reduction in the amount of moment-carrying
Eu$^{2+}$ ions.
