Abstract: The dates reported by Smith et al. in 1995 are the first 40 Ar- 39 Ar ages for the Jehol Group in western Liaoning, Northeast China, which provide a considerably improved age calibration of the lowermost Yixian Formation of the Jehol Group, and indicate that the whole group was deposited entirely within the Early Cretaceous time. However, because Smith et al.' s report mistook the dates of three epigenetic igneous rock samples as the ages of the upper and lower parts of the Yixian Formation, respectively (samples 5, 10, 4), the isotopic dating results, as a whole, are perplexing and have been greatly misunderstood since they were published in 1995. In fact, those dates for the Jehol Group by Smith et al. represent the 40 Ar- 39 Ar ages of at least four horizons: 122.9 ± 0.3Ma (volcanics underlying the Daxinfangzi Bed of the Yixian Formation), 122.1 ± 0.2— 122.5 ± 0.3Ma (Ershilipu Bed of the Yixian Formation), 121.5 ± 0.9— 121.6 ± 0.5Ma (volcanic breccias above the Jingangshan Bed of the Yixian Formation), 120.8 ± 0.4— 121.4 ± 0.7Ma (epigenetic igneous rocks overlying or intruding into the Yixian Formation).