By comparing UT1 determinations from the International Radio Interferometric Surveying intensive program of quasi-daily, single-baseline, 1-hour very long baseline interferometry (VLBI) measurement sessions with coincident, multibaseline 24-hour VLBI determinations, we estimate the overall accuracy of the intensive UT1 results, averaged over 5.4 years of data. to be ~52 μs, depending somewhat on the amount of error attributed to the multibaseline results. The largest error contributions are due to interpolated values for polar motion X and Y coordinates (≤38 μs), short-term variability in atmospheric propagation (~48 μs during summers but much less at other times), and extended brightness structures in the radio sources (~22 μs). Because the atmospheric effect is most pronounced during summers, the UT1 accuracy of the intensives is ~47 μs most of the year but degrades to ~67 μs during the third quarter of each year.