An X-ray crystallographic study of adducts of trialkylphosphine selenides with >1 equivalent of diiodine reveals that solid Bu^t₃PSeI₃ consists of cation pairs [(Bu^t₃PSe)₂I⁺]₂ intercalated between I₅^– layers and that solid R₂R′PSeI₇ (R = Bu^t or Prⁱ, R′ = Prⁱ) contains [R₂R′P–Se–I]⁺ cations with weak secondary I‥I interactions to polyiodide networks.