In the plasma experiment of H2+ ion injection into a magnetic mirror (MMII) at Fontenay-aux-Roses, Lorentz force trapping is observed when the residual gas pressure is low enough. The measured dissociation fraction of H2+ directly extracted from the ion source is relatively high, f ∼ 2×10−4, for an equivalent electric field maximum E = 3.2 × 104 V/cm. This result is attributed to the abundance of ions with high vibrational levels.