Two new orthophosphates KNi 4 (PO 4 ) 3 and RbMn 4 (PO 4 ) 3 , with tunnel structure, have been synthesized. Both compounds crystallize in the orthorhombic space group Pmnn, Z=4 whose parameters are a=6.152(2), b=16.214(2), c=9.484(3) for KNi 4 (PO 4 ) 3 and a=6.450(4), b=16.730(3), c=9.870(3) for RbMn 4 (PO 4 ) 3 . Their three-dimensional framework [M 4 P 3 O 1 2 ] ~ consists of edge-sharing trioctahedral units [M 3 O 1 3 ] and square pyramidal groups M 2 O 8 , and all of them are joined by the isolated PO 4 tetrahedra. This 3D lattice possesses large pseudo-hexagonal tunnels running along the c-direction where the alkali cations are located. Ionic conductivity in these materials as well as in the isostructural phases NaNi 4 (PO 4 ) 3 and KMn 4 (PO 4 ) 3 , was undertaken by impedance spectroscopy, and this behaviour is due to the alkali ions mobility through the tunnels. These results are analyzed on the basis of the structural characteristics and alkali cations sizes.