Characterisations of instantaneous mixing behaviours in fluidised beds with binary mixtures has many challenges.We studied the instantaneous mixing characteristics of binary mixtures in a 2-D quasi gas-solid fluidised bed using a capacitance probe method.This method enabled the quantitative assess-ment of instantaneous mixing behaviours,including mixing index,rate,and time.Three kinds of binary mixtures of similar size but different density were used for the transient and steady fluidisation exper-iments in a bubbling fluidised bed.The mixing curves of initially segregated binary mixtures were acquired,and the instantaneous characteristics for the entire process were analysed.An equation was proposed to describe the mixing process and predict the mixing degree over time.By comparing exper-iments with different working conditions,the effects of superficial gas velocity and density ratio on the mixing in the centre and at the wall of the fluidised bed were addressed.During the stable fluidisation stage,the axial concentration profiles of the mixtures were obtained,and the mechanisms of mixing were discussed.Results showed that the mixing level increased logarithmically with time and eventually reached a relatively stable value.Increasing the superficial gas velocity and reducing the density ratio promoted mixing and reduced mixing time.High superficial gas velocities tended to mix the mixtures well and quickly,regardless of the density ratio of the two components.