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The study of ionised molecular clusters provides an insight into ion-molecule chemistry; ubiquitous in atmospheric, flame and plasma processes. The neutral fluorobenzene-ammonia system, essentially unreactive in the gas phase, will react in solution under high temperature and pressure conditions. However, fluorobenzene$+$ will react, via nucleophilic substitution to form aniline$+$ and HF. In combination with the aquisition of REMPI spectra, we have observed the following reactions, post-ionisation, of fluorobenzene$+$-(NH$3$) (n=1,2) clusters: \vspace{1em} C$6$H$5$F$+$.(NH$3$)$2$ $⟶$ C$6$H$5$NH$2$$+$ \vspace{1em} \vspace{1em} C$6$H$5$F$+$.NH$3$ $⟶$ NH$3$$+$ + C$6$H$5$F \vspace{1em} While nucleophilic substitution satisfactorily accounts for the formation of aniline$+$, it is more difficult to rationalise the formation of NH$3$$+$ on thermodynamic grounds. By performing {\it ab initio} calculations we hope to be able to postulate a mechanism for this reaction.