This paper reports a novel topotactic exchange reaction of the ethylenediammonium intercalated mixed-valent vanadyl phosphate, (H3NCH2CH2NH3)0.5[V4+0.45V5+0.55O2PO4{P(OH)2}0.483], with monovalent cations at room temperature. Alkali metal ions and other monovalent cations (e.g. K+, Rb+, Tl+, NH4+) behave similarly, leading to intercalated layered vanadyl phosphates (basal spacing about 6.3-6.5Å). The amount of cations incorporated between the layers is essentially dictated by the charge present on the layers (equivalent to the amount of V4+ present). The present approach has enabled us to obtain a new phase, Tl0.45VOPO4·0.33H2O, under ambient conditions. All the layered solids show low-temperature magnetic behaviour similar to hydrothermally synthesised products. Treatment with silver ions resulted in a new silver vanadium(V) phosphate Ag3.5VP1.5O8 [monoclinic; a = 22.557(4), b = 7.973(3), c = 12.430(3) Å and = 112.26(2)°] coated with silver metal nanospheres (ca. 15 nm) which, on heating above 400 °C, yielded a 2:1 mixture of two thermodynamically stable phases, Ag2VO2PO4 and Ag3PO4.