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We determine the nature of coupled phonons in mixed crystal of Cs0.9(NH4)0.1H2AsO4 using inelastic light scattering studies in the temperature range of 5 K to 300 K covering a spectral range of 60–1100 cm−1. The phase transition in this system are marked by the splitting of phonon modes, appearance of new modes and anomalies in the frequency as well as linewidth of the phonon modes near transition temperature. In particular, we observed the splitting of symmetric (v1) and antisymmetric (v3) stretching vibrations associated with AsO4 tetrahedra below transition temperature (Tc* ∼ 110 K) attributed to the lowering of site symmetry of AsO4 in orthorhombic phase below transition temperature. In addition, the step-up (hardening) and step-down (softening) of the AsO4 bending vibrations (v4 (S9, S11) and v2 (S6)) below transition temperature signals the rapid development of long range ferroelectric order and proton ordering. The lowest frequency phonon (S1) mode observed at ∼92 cm−1 shows anomalous blue shift (∼12 %) from 300 K to 5 K with no sharp transition near Tc* unlike other observed phonon modes signaling its potential coupling with the proton tunneling mode.