  Let $\mathrm {SL}(2, \mathbb{H})$ be the group of $2 \times 2$ quaternionic
matrices with quaternionic determinant $1$. The group $\mathrm {SL}(2,
\mathbb{H})$ acts on the four-dimensional sphere $\widehat
{\mathbb{H}}=\mathbb{H} \cup \{\infty\}$ by the (orientation-preserving)
quaternionic M\"obius transformations: $$A=\begin{pmatrix} a & b \\ c & d
\end{pmatrix}: z \mapsto (az+b)(cz+d)^{-1}.$$ Using this action, the M\"obius
group may be identified with $\mathrm {PSL}(2, \mathbb{H})$. A quaternionic
M\"obius transformation $[A]$ is reversible if it is conjugate to its inverse
in $\mathrm {PSL}(2, \mathbb{H})$. Given an element $A$ in $\mathrm {SL}(2,
\mathbb{H})$, we provide a characterization in terms of the conjugacy
invariants of $A$ that recognizes whether or not $[A]$ is reversible.
Equivalently, this characterization detects whether $[A]$ is a product of two
involutions or not. Further, we revisit the algebraic characterization of the
quaternionic M\"obius transformations.
