Abstract: Lewis base adducts with selenenyl halides have been investigated experimentally and theoretically as models for interactions that might be relevant for the activity of Glutathione peroxidase (GPx).[1-3] Recently, we have shown that in donor adducts of selenenium ions with amines or phosphanes the formal donor-acceptor bond is covalent rather than dative.[4] Our findings suggested that in contrast to the description of adducts of the isolobal arsenium ions a description as selenenyl phosphonium and selenenyl ammonium ions appears more appropriate (Scheme 1). We also prepared compounds containing the selenenylphosphonium ion on a synthetic scale and studied their spectroscopic and chemical behaviour.[5] Here we describe ab-initio calculations exploring the reactivity of methyl substituted selenenyl phosphonium ions towards some simple nucleophiles and electrophiles.