The recombination of a pair of ions R⁺ and Z⁻ formed from @I03183@ of RZ. If the ions are paired as a @I03231-1@ and recombine without prior separation into a @I03231-2@ this is called 'internal ion-pair return': It is a special case of 'primary @G02603@'. If the ions are paired as a @I03231-2@ and form the covalent chemical species via a @I03231-1@, this is called 'external ion-pair return': It is a special case of 'secondary @G02603@'. When the covalent molecule RZ is reformed without direct evidence of prior partial @R05030@ or without other direct evidence of prior formation of a @I03231-1@, (e.g. without partial @R05030@ if the group R is suitably @C01057@) the internal ion-pair return is sometimes called a 'hidden return'. External (@M03989-1@) ion-pair return is to be distinguished from 'external (@M03989-2@) ion return', the (reversible) process whereby dissociated ions are converted into loose ion pairs: