A rapidly convergent series for computation of the interaction between two similar plane-parallel double layers is derived by expanding the interaction energy in power series of omega(0) (omega(0) < tanh (gamma0)/(64)). The series terms required to obtain the interaction energy with six significant 64 figures do not exceed 1 when the dimensionless surface potential gamma(0) of colloid particles is less than or equal to 2.00000 x 10. (C) 2004 Elsevier Inc. All rights reserved.