Ketenes 7, 14, and 17 have been generated in pyridine matrices and observed by infrared spectroscopy to react with pyridine at temperatures between 15 and 50 K to form ketene-pyridine ylides 12, 15, and 18. Azaheterocyclic ketenes 20, 23, and 26 dimerize or tetramerize at ca. 50 K, whereby one ketene molecule acts as a nucleophile toward the other, forming a transient ketene-imine ylide.