The axial ground state constants A₀, D^K₀ and H^K₀ of ¹³CH₃I were determined by combining lines from three perpendicular bands: the fundamental band v₆, its overtone 2v₆, and the related hot band 2v₆-v₆. Ground state combination differences, where ΔK = 3, were calculated for ten K value pairs from (3,0) to (12,9). As a result, values A₀ = 5·1742629(14) cm^-1, D^K₀ = 87·857(33) 10^-6 cm^-1, and H^K₀ = 4·44(21) 10^-9 cm^-1 were obtained.