Recent experiments and calculations have indicated that mutual collisions between excited 6 3P0, 6 3P1, and 6 3P2atoms may be important for the ion formation in low‐pressure Hg–noble‐gas discharges. We calculate various rates for electron‐impact and for excited‐atom‐collisional ionization for a range of conditions relevant to the Hg‐Ar fluorescent lamp (pAR=3 Torr,R=1.8 cm,Twall=20–80 °C, andI=0.2–0.8 A). In the analysis use is made of experimentally determined values for particle densities and electron temperatures. Deviations from a Maxwellian electron‐energy distribution, which substantially reduce the electron‐impact ionization rates, are taken into account. We find that 6 3P–6 3Pcollisions become even more important for the ion formation than electron impact at the higher Hg vapor pressures. Both Hg+and Hg+2will be formed. The 6 3P–6 3Pcollisions are indicated to be mainly responsible for the well‐known decrease in uv efficiency of the Hg–noble‐gas discharge at temperatures above 40 °C.