[en] Ytterbium-169 (¹⁶⁹Yb) is a promising new intermediate low-energy isotope for interstitial implantation. To date, no air kerma strength (S_K) standard for this source exists that can serve as a sound foundation for comparing various dose measurements and theoretical calculations. We have solved this problem by adapting the free air measurement technique of Goetsch et al, originally developed for ¹⁹²Ir. Using a 100 cm³ spherical ion chamber with NIST traceable external beam calibrations in a free air geometry, we have measured the air kerma strength of six different source batches (two type 6 batches, three type 8 batches, and one experimental high-intensity source). Room scatter corrections, derived from an empirical fit to the data (following Goetsch et al) and/or directly by Monte Carlo simulation, yielded identical results with a reproducibility of 1%. (author)