Trimethyloxonium fluoborate (CAS NO.: ), which is also known as Oxonium, trimethyl- tetrafluoroborate(1-), could be produced through the following synthesis route.

Freshly prepared triethyloxonium fluoborate (170 g., 0.90 mole) is dissolved in 500 ml. of anhydrous methylene chloride in a 1-l. three-necked flask equipped with a stirrer, gas-inlet tube, and drying tube. The reaction flask is immersed in an ice bath, the stirrer is started, and 138 g. (3.00 moles) of dry is passed into the solution from a tared cylinder over a period of about 2 hours. The reaction mixture is allowed to stand overnight at room temperature. An hour after the addition of dimethyl ether is complete, trimethyloxonium fluoborate begins to separate. The initially liquid product solidifies slowly.

The stirrer is replaced by a filter stick, and the supernatant methylene chloride is withdrawn from the crystalline mass of trimethyloxonium fluoborate; nitrogen is admitted through a bubbler during this operation to prevent atmospheric moisture from entering the flask. The crystals are washed with three 100-ml. portions of anhydrous methylene chloride. The flask is transferred to a dry box, and trimethyloxonium fluoborate is collected on a sintered-glass filter, dried for 2 hours in a vacuum desiccator at 25° (1 mm.), and bottled in a stream of dry nitrogen. The fluoborate is colorless; yield 114–124 g. (86–94%). Rapidly heated in an open capillary tube, it sinters and darkens, with decomposition, at 141–143°.