Abstract. Application of the specular reflection intensity was previously reported for the quantification of early dental erosion. Further development of the technique and assembly of the miniaturized pen-size instrument are described. The optical system was adjusted to fit into a handy device which could potentially access different positions in the oral cavity. The assembled instrument could successfully detect early erosion progression in both polished (n ¼ 70) and native (n ¼ 20) human enamels. Different severities of enamel erosion were induced by varying incubation time of polished enamel in 1% citric acid (pH ¼ 3.60, 0.5 to 10 min), while the native incisors were treated in the commercial orange juice (Tropicana Pure Premium ® , pH ¼ 3.85, 10 to 60 min). The instrument provided a good differentiation between various severities of the erosion in vitro. The size of the measurement spot affected the erosion monitoring in native enamel (human incisors). The erosion measurement in the 0.7-mm (diameter) cervical spots showed systematically lower reflection intensities compared with the analysis of central and incisal small spots. The application of larger spot areas (2.3 mm) for the erosion monitoring revealed no effect (p > 0.05) of the spot position on the reflection signal. High variation of the teeth susceptibility toward in vitro erosion was detected in native enamel.