We test the hypothesis that El Ni¿o/Southern Oscillation (ENSO) events are caused or enhanced by volcanic aerosol perturbations using two methods: 1) A case-by-case comparison of the 16 strongest El Ni¿o events of the last 150 years with the characteristics of the largest concurrent volcanic events, and 2) Comparison of the timing of strong El Ni¿o events and the independently determined record of stratospheric optical depth (&tgr;) perturbations. Many eruptions that occurred near times of strong El Ni¿o years produced small amounts of stratospheric aerosols, in many cases the relative timing of the two events argues against triggering. The correlation of 4 of 11 peaks in global stratospheric optical depth (&tgr;≥0.025) within 3 years of strong El Ni¿o events over the last 150 years is what would be expected by chance. Moreover, five strong El Ni¿os occurred between 1915 and 1960, when the stratosphere was largely free of volcanic aerosols. Of the three strongest tropical volcanic aerosol perturbations that coincided with or were followed by strong El Ni¿os in the period studied (Krakatau in 1883, El Chich¿n in 1982, and Pinatubo in 1991), the two modern ones occurred after the earliest SST warming of the El Ni¿o events. The coincidence of these two phenomena indicates no causative relationship.¿ 1997 American Geophysical Union