Abstract. We report laboratory observations of H₂SO₄ and H₂O homogeneous nucleation made under atmospherically relevant conditions ([H₂SO₄] of 10⁶ – 10⁷ cm⁻³ and 287 K). Our observations show that nucleation takes place at [H₂SO₄] of 10⁶ – 10⁷ cm⁻³, as observed in the atmosphere. The slope of nucleation rate (J) vs. [H₂SO₄] ranges between 4–6, consistent with thermodynamic predictions of neutral H₂SO₄ clusters, but is higher than those observed in the atmosphere. These results indicate that ternary aerosol precursors are needed to reduce the slope to 1–2 in the atmosphere. This study also discusses the effects of experimental parameters on laboratory observation results, in order to properly interpret the experimental data.