(Liquid + liquid) phase equilibria of the binary closed-loop system {xn-C3H7(n-C4H9)(3)N+I- +(1 - x)H2O) were investigated at pressures up to 200 MPa. The measurements were performed in an optical high-pressure cell with magnetic stirring according to the synthetic method. Data for p(T) isopleths are reported. Isobaric T(x) phase diagrams are generated which exhibit closed-loop behavior. The pressure dependence of the closed loop indicates the presence of overlapping low-and high-pressure immiscibility regimes, resulting in a tube-like shape of the (p, T, x)-surface of immiscibility. Consequences for thermodynamic functions are discussed in relation to available experimental data for lower homologues of the tetra-alkylammonium salt series, which are completely miscible with water.