Purpose: The purpose of this study was to evaluate, in situ, the initial adhesion of microorganisms to as well as the surface roughness and chemical composition of ProBase Hot (Ivoclar Vivadent), a hard acrylic resin, and Vertex Soft (Vertex-Dental), an acrylic-based soft liner, used in removable dental prostheses. Materials and Methods: Equal sized disks of ProBase Hot and Vertex Soft were prepared and polished according to the recommended procedures for clinical use. Two disks of each material were mounted in individual oral splints and exposed for 4 hours to the oral cavities of 15 participants. The microbial adhesion to each material's surface was measured with the pour plate technique using rich and selective growth media. Statistical analysis was performed using the Student t test. Scanning electron microscopy and chemical composition analyses obtained through electron probe radiographic microanalysis of sample disks also were performed. Results: In comparison to ProBase Hot, Vertex Soft presented higher microbial adhesion, namely regarding total aerobes, anaerobes, streptococci, and mutans streptococci (P .05). Also, Vertex Soft presented higher surface roughness. Differences in the chemical composition of the two materials also were found. Conclusions: The Vertex Soft liner has been found to be more susceptible to microbial adhesion than the acrylic resin ProBase Hot, probably due to its greater surface roughness. The application of Vertex Soft liner to a hard denture base may lead to a higher risk of oral and systemic infections for patients, highlighting a greater need for plaque control, especially for more susceptible individuals.