  Heavy ion ratio abundances in Solar Energetic Particle (SEP) events,
e.g.~Fe/O, often exhibit decreases over time. Using particle instruments on the
ACE, SOHO and STEREO spacecraft, we analysed heavy ion data from 4 SEP events
taking place between December 2006 and December 2014. We constructed 36
different ionic pairs and studied their time evolution in each event. We
quantified the temporal behaviour of abundant SEP ratios by fitting the data to
derive a decay time constant $B$. We also considered the ratio of ionic
mass--to--charge for each pair, the $S$ value given e.g.~for Fe/O by $S_{\rm
Fe/O} = (M/Q)_{\rm Fe}\big/(M/Q)_{\rm O}$. We found that the temporal behaviour
of SEP ratios is ordered by the value of $S$: ratios with $S>1$ showed
decreases over time (i.e.~$B<0$) and those with $S<1$ showed increases ($B>0$).
We plotted $B$ as a function of $S$ and observed a clear monotonic dependence:
ratios with a large $S$ decayed at a higher rate. A prominent discontinuity at
$S=2.0$ (corresponding to He/H) was found in 3 of the 4 events, suggesting
anomalous behaviour of protons. The X/H ratios often show an initial increase
followed by a decrease, and decay at a slower rate. We discuss possible causes
of the observed $B$ versus $S$ trends within current understanding of SEP
propagation.
