A dual-beam thermal lens optical fiber spectrometer was developed and tested with aqueous solutions of the rare-earth ion Nd³⁺. An argon ion pumped dye laser beam was delivered to a sample cuvette through optical fibers of 100, 200, and 400 µm core diameter. The thermal lens signal was detected with a He-Ne laser guided to a photodiode detector by an optical fiber of 200 µm core diameter. Theoretical values for the optical aperture, excitation and probe beam radii, and distances from the sample to the probe beam waist and to the aperture-detector were calculated and compared to experimental results. Theoretical thermal lens enhancement factors that take into account the radius of the excitation beam show good agreement with the experimental values. A detection limit of 7 x 10^-8 M and an enhancement factor of 0.9 were obtained for an Incident power of 25 mW and a power density of 31 W cm^-2.