The Sauter mean diameter (d(32)) of bubbles was characterized for a gas-liquid system in a laboratory Jameson-type flotation cell with focus on the size variation in the uprising path of the bubbles in the riser of the flotation cell. Methyl isobutyl carbinol (MIBC) was used as frother for bubble stability. The effect of MIBC concentration, sampling height in the riser, gas flow rate (J(g)) and liquid flow rate (J(I)) in the downcomer on d(32) was investigated. The d(32) significantly decreased with increasing MIBC concentration until the Critical Coalescence Concentration (CCC), above which the d32 was almost constant at 0.645 mm. CCC95, CCC90 and CCC85 were calculated to be 0.059, 0.046 and 0.038 mmol/L, respectively for a Jg of 1.32 cm/s and J(I) of 11 cm/s. Four frother concentrations covering these three values were selected for detailed studies. The size variation of bubbles was related to the Reynolds number (Re) in the downcomer, where the Re was influenced by J(I) and J(g). Bubble size increased with the sampling height in the riser at MIBC concentrations below CCC95. This bubble size decreased with the Re and, for all the MIBC concentrations used in this investigation, it reached a critical value, even at MIBC concentrations below the CCC85. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.