Removal of tars produced during biomass gasification continues to be a technical barrier confronted by developers of commercial thermochernical conversion systems. Quantitative measurement of tar in the synthesis gas (syngas) stream is important to assess the effectiveness of cleanup and conditioning processes and verify the suitability of the cleaned syngas for its intended downstream use (e.g., catalytic conversion to liquid fuels, hydrogen recovery, or electricity production). In an effort to advance the art of gasifier tar measurement and address some limitations of traditional impinger sampling, we have investigated the use of a molecular-beam mass spectrometer (NIBMS) sampling system as an alternative method for quantifying real-time tar concentrations in biomass gasifier-derived syngas. The 0.5 ton/day pilot-scale biomass gasification system of the National Renewable Energy Laboratory (NREL) has enabled direct comparisons between NIBMS sampling and replicate impinger sampling during continuous operations. The results have shown some systematic differences between the methods, although they do appear correlated. Using a synthetic tar mixture, as well as actual corn-stoverderived syngas, experiments were carried out to compare the accuracy of the two methods. Both methods demonstrated good reproducibility, but the NIBMS measurements appear to be more accurate. Tar concentrations determined from impinger sampling averaged 11-21% lower than expected, depending upon the compound. Average MBMS measurements were within 6% of the known values, demonstrating that the MBMS can be used to improve quantitative, continuous, real-time monitoring of gasifier tar.