Measurements are reported of rate constants for electron attachment to ${\mathrm{C}}_{6}{\mathrm{F}}_{5}X$ $(X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$ and thermal electron detachment from ${\mathrm{C}}_{6}{\mathrm{F}}_{5}{\mathrm{Cl}}^{\ensuremath{-}}$ over the temperature range $300--550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $133\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}$ of He gas in a flowing-afterglow Langmuir-probe apparatus. This is the first case we know of where the parent anion has sufficiently low electron detachment energy that detachment (from ${\mathrm{C}}_{6}{\mathrm{F}}_{5}{\mathrm{Cl}}^{\ensuremath{-}}$ in this case) has been observed in competition with a channel for dissociative electron attachment yielding a thermally stable anion (here, ${\mathrm{Cl}}^{\ensuremath{-}}$). Because of this competition, it is shown that a simple mass spectrometric determination of the product branching fractions at long times will lead to erroneous results at elevated temperatures. The electron density profiles provide evidence for a new plasma decay process involving the detaching and nondetaching anions trapped in the space charge field of the positive ions. Electron attachment rate constants were found to be $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, and $2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Cl}$, ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Br}$, and ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{I}$, respectively, estimated accurate to $\ifmmode\pm\else\textpm\fi{}25%$ except for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{I}$, where there is $\ifmmode\pm\else\textpm\fi{}30%$ uncertainty. Rate constants for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Cl}$ changed little over our temperature range, while those for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Br}$, and ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{I}$ increased with temperature. Electron detachment occurred only for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}{\mathrm{Cl}}^{\ensuremath{-}}$ in our temperature range. Detachment rate constants were immeasurable at room temperature but approached $4000\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at $550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. From these data the electron affinity (EA) for ${\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Cl}$ was determined, EA $({\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Cl})=0.75\ifmmode\pm\else\textpm\fi{}0.08\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. G3(MP2) calculations (based on M\o{}ller-Plesset perturbation theory) were carried out for the neutral and anion and yielded $\mathrm{EA}({\mathrm{C}}_{6}{\mathrm{F}}_{5}\mathrm{Cl})=0.728\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.