Insulating oxides are among the most abundant solid materials in the universe ^1–3 . Of the many ways in which they influence natural phenomena, perhaps the most consequential is their capacity to transfer electrical charge during contact ^4–10 —which occurs even between samples of the same oxide—yet the symmetry-breaking parameter that causes this remains unidentified ^11,12 . Here we show that adventitious carbonaceous molecules adsorbed from the environment are the symmetry-breaking factor in same-material oxide contact electrification (CE). We use acoustic levitation to measure charge exchange between a sphere and a plate composed of identical amorphous silicon dioxide (SiO ₂ ). Although charging polarity is random for co-prepared samples, we control it with baking or plasma treatment. Observing the charge-exchange relaxation afterwards, we see dynamics over a timescale of hours and connect this directly to the presence of adventitious carbon with time-of-flight mass spectrometry, low-energy ion scattering and infrared spectroscopy. Going further, we confirm that adventitious carbon can even determine charge exchange among different oxides. Our results identify the symmetry-breaking parameter that causes insulating oxides to exchange charge in settings ranging from desert sands ⁴ to volcanic plumes ^5,6 , while simultaneously highlighting an overlooked factor in CE more broadly.