Diamondoids are sp³-hybridized carbon nanomaterials that hold promise for application in biotechnology, medicine, and opto- and nanoelectronics. While difficult or even impossible to realize by conventional organic synthesis methods, the synthesis of larger diamondoids was demonstrated by plasmas generated in supercritical fluids, however, the reaction mechanisms that lead to their formation are not clear. Here, we investigated the synthesis of diamondoids from adamantane and present a simple model that predicts the reaction rates of diamantane, triamantane, and tetramantane. The diamondoids were synthesized by micro dielectric barrier discharges arranged in arrays, generated at frequencies of 7 kHz and applied voltages of 5 − 10 kV_p−p in supercritical xenon (pressure 5.56 − 8.22 MPa, temperature 290.0 − 310.2 K). The collected samples were analyzed and quantified by gas chromatography - mass spectrometry measurements. The reaction yields of diamantane increase with increasing precursor density and longer reaction times. Experimental data also suggests a stepwise reaction of higher diamondoids from lower diamondoids, and predicts lower reaction rates for higher diamondoids when compared to lower diamondoids.