Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, A(D)/A(R) ratio between 0.111-1.000, total liquid volume V-T=(1.189-2.446).10(-3) m(3)) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, A(D)/A(R) ratio of 0.1225 and 0.040 m, total liquid volume, V-T=(0.144-0.170) m(3)) were used.The influences of reactor geometry characterized by some parameter as : A(D)/A(R) ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters : gas holdup and liquid circulation velocity were analysed.The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible.The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift reactors with almost complete phase separation at the top.An original dimensionless correlation for gas holdup prediction involving superficial velocities of gas and liquid, cross sectional areas, dispersion height, riser diameter, as well as Froude number, was obtained.