A seismic refraction survey was carried out over the Central Coalfield Syncline in the Midland Valley of Scotland during May 1984, to determine depth to sub-Carboniferous basement and to locate intra-basement contrasts and structure. Three refractors were identified and mapped. The shallowest is thought to be the top of the Lower Old Red Sandstone, so that the top layer of the crustal model defines the Carboniferous and Upper Old Red Sandstone which varies in thickness from 1.5 to 3.5 km. Deeper refractors are interpreted as the top of the crystalline basement and an intra-basement horizon. The depth of the top basement refractor limits the combined thickness of Lower Old Red Sandstone and Lower Palaeozoic sedimentary rocks to between 0 and 5 km. No obvious correlations with the Bathgate gravity and magnetic anomaly are found in the seismic data. Gravity modelling based on the seismic interpretation suggests that the likeliest cause of the anomaly is thick extrusive basic igneous rocks of Carboniferous and/or Lower Old Red Sandstone age.