A simple, rapid and effective solvent extraction separation of rare earth elements (REEs) has been developed for their determination in geological samples such as rock, soil and core samples by inductively coupled plasma optical emission spectrometry (ICP-OES). Rare earth elements form neutral complexes with the bi-dentate ligand, 2,3-dihydroxynaphthalene (2,3-H₂ND), and are extracted into ethyl acetate at an alkaline pH of 9–10. All these elements are then quantitatively stripped off into the aqueous phase using dilute hydrochloric acid and then determined by ICP-OES. Different variables such as pH of the solution, reagent concentration, solvent, stripping solution, diverse ions, etc. affecting the simultaneous quantitative extraction of lanthanides followed by their determination using ICP-OES have been studied in detail. The method has been validated by analyzing a set of certified reference materials of rock and soil samples such as SY-2, SY-3, GSP-2 and SO-2. The results obtained by the proposed method for a set of silicate rock and core samples are compared to those obtained by other standard separation techniques such as oxalate co-precipitation using calcium as a carrier. The precision (as % RSD) of the proposed method varied over a range of 1 to 10% depending on the concentration of the lanthanides. The developed method involving simultaneous separation and determination of individual lanthanides by ICP-OES is an improvement on the existing method, where only total lanthanides were determined colorimetrically using arsenazo-III, and does not require any counter cation. The proposed method is comparatively more environment friendly.