The proposed relationship between flotation rate constant k and bubble surface area flux S-b for the pulp zone, namely k = PSb, where P is the "floatability factor", is tested at the microscale. Bubble size D-b was measured by an adaptation of the McGill bubble size analyzer. The range in S-b, up to 50 s(-1), is comparable to full-scale machines. Standard statistical procedures were used to asses the plausibility of the model for the two materials tested, NiO-20 μ m and galena-37 μ m giving P ≈ 1.73 x 10(-3) and ≈ 2.82 x 10(-3), respectively, comparable to available values. The data, however, cannot unambiguously distinguish among the various relationships of the form k ∝ 1/D-b(m) where m may range up to 2. © 2004 Elsevier Ltd. All rights reserved.