The pK(a) value of an amine aqueous solution is directly related to the rate,of its chemical reaction with CO2. A quick and accurate pKa, predicted Method can provide a shortcut for efficient screening of CO2 absorbents. The original and updated Perrin-Dempsey-Serjeant (PDS) pencil-and-paper group-addition methods were used to predict the pK(a). The errors in pK(a) predictions of some cyclic, tertiary, and multihydroxy amine solutions were larger than that of the linear primary and secondary amines whether the original PDS or the updated PDS method was used. Another computer-free group-addition method (called QSSG) was proposed to predict the pK(a) values of 73 CO2-relevant alkanolamines/amines and cyclic amines, including piperazine and morpholine derivatives. Intramolecular hydrogen bonding, hyperconjugation, solvent, and steric effects were analyzed. Six extra parameters were added, and 10 parameters were changed to improve the accuracy of the predicted pK(a) values, based on a linear free energy relationship, of a training set of 73 amines for industrial CO2 capture. The root-mean-square errors (rms) were 0.13 pK(a) units for a set of 73 relevant industrial CO2-capture amines. The rms errors in the pK(a) values of nine multihydroxy amines were 0.45 for the original PDS, 0.28 for Sumon's updated PDS method, and 0.12 for our QSSG method; for-cyclic tertiary amines the corresponding values were 0.47, 0.22, and 0.12, respectively. The 10 amines outside the training,agreed well with the experimental results with QSSG predictions.