Raman spectra and SERS of 1H-2(Phenylazo) imidazole (PaiH) adsorbed on silver nano particles are reported. Monomolecular layer is formed at a concentration of 5 x 10(-6) M. A trans-to-cis isomerisation of PaiH is suggested by the cis-signature peak at 570 cm(-1). In absorption spectra a single pi-pi* band at 358 nm is observed at higher concentrations whereas the pi-pi*/and the n-pi* bands appear at 370 nm and 456 nm, respectively, at a concentration of 5 x 10(-6) M. This is in support of the hypothesis of trans-to-cis-isomerisation with lowering of concentration. DFT calculations are shown. (C) 2013 Elsevier B. V. All rights reserved.