The influence of pentavalent arsenic and phosphorus on crystal chemical features of the feldspar framework was studied on the minerals belonging to the sanidine–filatovite solid-solution series from fumarolic exhalations of the Tolbachik volcano (Kamchatka, Russia). The reported feldspars demonstrate the first example of natural continuous solid solution between silicate and arsenate. The content of As₂O₅ varies from 0.0 to 45 wt% along with the highest phosphorus content found in natural feldspars: up to 5 wt% P₂O₅. The major substitution schemes in the studied feldspars are Al³⁺ + M⁵⁺ → 2Si⁴⁺ and M²⁺ + M⁵⁺ → Al³⁺ + Si⁴⁺, where M²⁺ = Zn and Cu and M⁵⁺ = As and P. The crystal structures of four samples with empirical formulae K_0.95[(Si_2.98Al_0.93Fe_0.08As_0.02)_Ʃ4.01O₈] (space group C2/m), (K_0.91Na_0.06Ca_0.03)_Ʃ1.00[(Si_2.81Al_1.14As_0.03P_0.03Fe_0.01)_Ʃ4.02O₈] (C2/m), K_0.82[(Al_1.92As_1.11Si_0.89Zn_0.05Fe_0.03Cu_0.02P_0.01)_Ʃ4.03O₈] (I2/c), K_0.99[(Si_1.87Al_1.51As_0.53P_0.05Fe_0.02Cu_0.01)_Ʃ3.99O₈] (I2/c) were studied. The phase transition in the sanidine–filatovite solid-solution series is C2/m → I2/c. This is caused by the change of the Al:(Si + As) ratio from 1:3 in sanidine to 1:1 in filatovite along with significant ordering of Al, Si and As in the framework.