Electrochemical sensors made with layer-by-layer (LbL) films of functionalized multi-walled carbon nanotubes (MWCNTs-COO-) and polyaniline (PAni) deposited onto ITO substrates were used to detect 2-chlorophenol. Concentrations down to ppm level could be detected with square wave voltammetry (SWV), owing to synergistic effects between PAni and nanotubes, whose molecular-level interactions in the LbL films were inferred using FTIR and Raman spectroscopy. Significantly, the Raman spectra of MWCNTs-COO-/PAni LbL films indicated that the conducting properties of PAni were preserved upon adsorption on the ITO substrates, with homogeneous nanocomposites being formed according to Scanning Electron Microscopy (SEM) images.