The Silurian bituminous sandstones (SBS) in the Tarim Basin, China are important basin-wide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the 40Ar/39Ar step wise heating method. The age spectra, 39Ar recoil loss and their controlling factors were investigated systematically. The 40Ar/39Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite (I/S) with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated 40Ar/39Ar total gas ages (UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older (late Caledonian-early Hercynian) around the basin margin, younger (late Hercynian) in the basin centre, and the youngest (middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the 39Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated 39Ar recoil losses in this study are in the range from 7% to 51%. The 39Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers (IR) of the I/S, and smectite layer content (SLC) of the samples. We conclude that due to significant 39Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. 39Ar recoil losses during 40Ar/39Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers (IR<30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.