Theophylline matrices containing mixtures of xanthan gum and hydroxypropyl methylcellulose were studied for their sustained release properties. The following factors that might influence the drug release were also investigated: amounts of polymers, mixing ratios of xanthan gum and hydroxypropyl methylcellulose, types of fillers, dissolution media having different ionic strengths or pH, and compression forces employed to prepare the matrices. The matrices prepared with the increasing amounts of polymers (10, 15 and 20%) exhibited the decreased drug release rate. The amount of drug releases from matrices which were incorporated with soluble-filler (spray dried lactose) were greater than that from the matrices contained insoluble-filler (dibasic calcium phosphate). The different compression forces (1000, 2000 and 4000 lbs) slightly affected the drug releases. The matrices containing hydroxypropyl methylcellulose alone showed the initial rapid drug releases. However, the replacements of hydroxypropyl methylcellulose with xanthan gum at the ratios of 3:7, 5:5, 7:3 and 10:0 could reduce the initial rapid drug releases. The drug releases from matrices containing mixtures of xanthan gum and hydroxypropyl methylcellulose were influenced by the ionic strengths of dissolution media and the viscosities of hydrated gel layer around the matrices. The difference in drug release characteristics could be explained on the basis of the different swellings and erosions of matrices. The drug release mechanisms of matrices depended on the compositions of the matrices and the conditions of dissoulution media. The drug releases were controlled by the diffusion and erosion mechanisms.