The optical properties of multi-layer InAs/InP quantum wires (QWRs) with two different spacer thicknesses have been investigated by means of room temperature surface photovoltage (SPV) and photoluminescence (PL) spectroscopies, combined with empirical tight binding electronic structure calculations and structural data. The SPV and PL spectra reveal several features, which energy positions are in good agreement. They have been ascribed to excitonic transitions, which take place in the QWR families with heights differing by an integer number of monolayers. Comparing the experimental results with the theoretical ones, we have estimated the QWR family heights and the average atomic concentration of phosphorus in the QWRs. From the simultaneous analysis of the SPV amplitude and phase spectra, based on our vector model for SPV signal representation, a deeper understanding of the SPV results and of the mechanisms of carrier separation in the sample is obtained.