Knowledge of the shallow velocity structure is very important to understand the possible site effects which could greatly contribute to the irregular damage distribution observed in Lisbon for the 1755 earthquake. Taking advantage of small charge blasts in the Tagus River bed in front of Lisbon, seismic records were obtained in a scattered array extending to a 8.5 km distance from the shotpoint. Geological considerations allowed the grouping of traces in wide profiles for three zones; West, East and South. The presence of a distinct group of Rayleigh waves made possible the joint modelling of first arrivals and surface waves for the West and East zones. The P and S-wave velocity models inferred show the existence of two low velocity layers (in the West zone), which is in agreement with the known geology for the area. The East zone structure could not be resolved in detail from refraction data only, but surface wave analysis allow the identification of the first low velocity layer. For the South zone, the study of first arrivals revealed a structure similar to the West zone, although velocity values and interface depths are slightly different. The interpretation of these models was done according to the age and lithology of the different geological formations. The final models obtained represent a significant improvement in detail over previous regional studies for the Tagus Valley basin (Mendes-Victor et al., 1980) and over the preliminary interpretation of this data presented in Mendes-Victor (1987).