Polish rolled steel (16HG) and 15HG sheets were carburized (i) 20 min in a stream of (4:1 v/v) C2H4-H-2 at 5, 50 or 1000 mbar/950degreesC or (ii) over 25-950degreesC at 10degreesC/min with 40 mL/min C2H4-H-2 At 600degreesC, C2H4 began to be absorbed and H 2 to evolve, at 650-750degreesC, C was formed and, over 750-950degreesC, C = C broke to yield CH4 and/or propyne. MS disclosed also cyclo- and butadiene, and C6H6. Pathways to atomic, amorphous, dissolved, and fibrous C and to metal carbides were shown, confirmed by TOF-SIMS spectra. Topical CmHn(m, n = 1, 3; 1, 2; 1, 1; 2, 3; 2,5; 3, 5) and metal atoms (B, Na, K, Ca, Mn, Cr) were nearly identical regardless of pressure. H-2 slowed down the carbon deposition rate and suppressed secondary reactions.