A total of 63 new gravity stations was established in the Reydarfj¿rdur area and around the Iceland Research Drilling Project (IRDP) drill site. Both previous gravity stations and the new ones have been incorporated into a new Bourguer anomaly map. The main features of the area are a strong W-E gradient in the vicinity of the IRDP drill site, an extended gravity minimum a few kilometers east of Reydarfj¿rdur village, and a gravity maximum at the Reydarfj¿rdur volcanic center. These features correlate with dike density distribution. Dikes, however, can explain only 10% of the observed gravity amplitudes. Relief on a zone of density gradient observed in flows from the IRDP core and extrapolated to about 1 km below the bottom of the hole has been modeled by assuming constant density sublayers. Relief amplitudes of the order of 1 km were necessary to explain the anomalies. We conclude that the main source of the gravity anomaly, at least in the vicinity of the drill site, lies above the depth of the layer 2/3 boundary or transition zone indicated for the general area by refraction seismics.