p-Nitrophenylchlorocarbene reacted reversibly with diethyl ether, di-n-propyl ether, or tetrahydrofuran (THF) to form O-ylides, which were visualized by their UV-visible spectroscopic signatures. Equilibrium constants (K(eq)) Were determined spectroscopically and ranged from 0.10 M(-1) (di-n-propyl ether) to 7.5 M(-1) (THF) at 295 K. Studies of K(eq) as a function of temperature afforded Delta H degrees, Delta S degrees, and Delta G degrees for the di-n-propyl ether and THF/O-ylide equilibria. Delta H degrees was favorable for ylide formation, but Delta S degrees was quite negative, so that Delta G degrees s for the equilibria were small. Electronic structure calculations based on density functional theory provided structures, spectroscopic signatures, and energetics for the carbene/ether O ylides.