Two cation sites in a unit cell with varying cation (NH4+) symmetry are rare, and Rb-1.36(NH4)(1.64)Cr(O-2)(4) is one such compound with distinct cation sites. We report a systematic X-ray diffraction study using both single crystals and powder samples of Rb-1.36(NH4)(1.64)Cr(O-2)(4), changes in the lattice parameters and symmetry as a function of temperature at a wide range (10-300 K). Pseudo octahedral NH4+ ions at cation sites, N2, are involved in N-H center dot center dot center dot O hydrogen bond to yield a continuous second order phase transition in Rb-1.36(NH4)(1.64)Cr(O-2)(4) in contrast to a first order step like irregular pattern in its parent compound (NH4)(3)Cr(O-2)(4). High field EPR studies of Cr5+ at a temperature range 10-300 K revealed a distortion in Cr(O-2)(4 )groups in the plane (001) suggesting strong N-H center dot center dot center dot O hydrogen bonding between (NH4) ions and Cr(O-2)(4) groups.