As bond strength of currently available adhesive systems in caries-affected teeth dentin on primary tooth dentin was not well known, the bond strength of two bonding systems (PQI and OptiBond Solo Plus) was evaluated on caries-affected and sound primary molar tooth dentin and observed the micromorphology of the adhesive-dentin interfaces. By grinding both the sound (n = 30) and caries-affected (n = 30) approximal surfaces of teeth, flat dentin surfaces were obtained. The prepared surfaces were bonded with one of the each adhesive systems and a composite resin. After storing the bonded specimens in water at 37 °C for 24 h, the samples were sectioned and the bond strength of the adhesive systems was tested by the microshear test method. The data were statistically analysed. Microshear bond strengths of PQI group for caries-affected and sound primary tooth dentin were 9.43 ± 2.44 (MPa) and 9.32 ± 2.95 (MPa) (P > 0.05), respectively, and the bond strengths of OptiBond Solo Plus group for caries-affected and sound primary tooth dentin were 15.33 ± 3.59 (MPa) and 11.96 ± 2.30 (MPa) (P < 0.05), respectively. Micromorphological features between sound and caries-affected dentin were similar in both PQI and OptiBond Solo Plus groups. Both the adhesives showed significantly different bond strengths in caries-affected dentin but showed similar bond strengths in sound dentin. © 2007 The AuthorsJournal compilation © 2007 BSPD, IAPD and Blackwell Publishing Ltd.