The thermochemistry of several ion-molecule clustering reactions of HCNH + , HCNH + (N 2 ) and HCNH + (CH 4 ) ions with C 2 H 2 was determined from chemical equilibrium measurements. Inorder to better understand the chemistry of Titan, a gas mixture containing 97% N 2 , 3% CH 4 anda small amount of 0.005% C 2 H 2 was irradiated by α-particles in the temperature range 140–320 K and analysed by high-pressure mass spectrometry. The enthalpy and entropy changes for thereactions HCNH + (C 2 H 2 ) n +C 2 H 2 +N2←→HCNH + (C 2 H 2 ) n +1 +N 2 (n=0–2), HCNH + (C 2 H 2 ) n +N 2 +N2←→HCNH + (C 2 H 2 ) n (N 2 )+N 2 (n=1, 2), HCNH + (C 2 H 2 ) n +CH 4 +N2←→HCNH + (C 2 H 2 ) n (CH 4 )+N 2 (n=1, 2), HCNH + (C 2 H 2 ) n (N 2 )+C 2 H 2 +N2←→HCNH + (C 2 H 2 ) n +1 (N 2 )+N 2 (n=0, 1) have been determined. For the HCNH + (C 2 H 2 ) n series,the absolute enthalpy change decreases monotonically from 12.1 to 2.8 kcal mol −1 for n=1 to 3,showing no noticeable effect concerning the filling of shells. The nature of the bonding remainsmainly electrostatic. For the HCNH + (C 2 H 2 ) n (X) series with X=N 2 or CH 4 , the free energychanges ΔG° were calculated for T=90 K and 165 K. For both temperatures, ΔG°≈0 forreplacement of N 2 by CH 4 for n=0 to 2 wheareas ΔG° for the substitution of N 2 or CH 4 by C 2 H 2 are negative for n=0, 1. The ligand-exchange reaction leading to the formation of HCNH + (C 2 H 2 )and HCNH + (C 2 H 2 ) 2 could exist in the lower Titans atmosphere. Paths involving HCNH + (C 2 H 2 )(N 2 ) or HCNH + (C 2 H 2 )(CH 4 ) and yielding to HCNH + (C 2 H 2 ) 2 ions could also be possible and thuscould lead to heavier organic-nitrogen compounds and eventually large aerosols.