In this study, the solvent-swelling processes of coal particles of Zao Zhuang coal and Taiheiyo coal were investigated using a novel orthogonal microscopy image analysis technique proposed recently by our laboratory. The overshoot swelling and slow-climbing swelling are characteristic of Zao Zhuang coal and Taiheiyo coal particles in pyridine at ambient temperature, respectively. Super-Case II diffusion is dominant in Zao Zhuang coal particles and anomalous diffusion in Taiheiyo coal. The present study shows that the slow-climbing-type solvent-swelling processes of Taiheiyo coal particles can be described using the semiempirical model proposed by Ritger and Peppas. However, overall solvent-swelling processes with an overshoot feature in many coals could not be simulated well by the model. In addition, we observed that both the diffusion mechanism exponent n and proportionality constant k in the model for the two coals have characteristic statistical distributions. The distributions of n and k for Zao Zhuang coal are normal, and those of Taiheiyo coal are doubly exponential. On the basis of these observations, the connotation of the parameters, k and n, in the Ritger and Peppas model was expanded in view of a statistical distribution. On the other hand, based on the analysis of swelling features of Zao Zhuang coal particles in pyridine at ambient temperature, a series of new phenomenological models, a second-order system with a unit-step forcing function in nature, was used to describe the solvent-swelling processes of Zao Zhuang coal particles with or without overshoot features. The simulated overall processes of the coal particles using the new models agree very well with the measured ones and suggest that the viscoelastic properties of pyridine-dilated Zao Zhuang coal, the main features of solvent-swelling and/or solvent transport of the coal particles, can be well characterized by second-order system with the denominator tau(2)s(2) + 2 xi tau s + 1 and a unit-step forcing function (1/s).