The effect of ammonium sulfate (AMS) on efficacy, absorption, and translocation of glufosinate in various weed species was investigated in greenhouse and field experiments. Adding ammonium sulfate at 20 g L−1 increased the efficacy of glufosinate on Echinochloa crus-galli, Setaria faberi, and Abutilon theophrasti, but not on Amaranthus rudis or Chenopodium album. AMS increased foliar absorption of 14C from 14C-glufosinate to the greatest extent in A. theophrasti and S. faberi and to the least extent in C. album. AMS increased the translocation of 14C out of the treated leaf 24 h after treatment in A. theophrasti and S. faberi, but not in C. album. These results suggest that AMS can increase the efficacy of glufosinate on A. theophrasti and S. faberi by increasing foliar absorption and subsequent translocation of glufosinate.