A totally new product, by the name Scotchbond 2 (3M Co) appeared recently in the field of bonding agents. Grace to its improved formula, the manufacturers claim that both the bond strength and the sealing ability of the material, render it trustful for use in both anterior and posterior composite resin restorations. The purpose of this in vitro investigation was to determine the microleakage pattern around slot type cavities, restored with posterior composite resin (P-50, 3M Co) using two different generations of bonding agents. Two types of cavities were prepared in sound premolars extracted for orthodontic reasons. In the first group (10 teeth) the cervical wall was located in the enamel, above the cementoenamel junction and in the second group (10 teeth) the cervical wall was located below the CEJ. The enamel surrounding the cavity margins was etched using 37% orthophosphoric acid gel, rinsed and dried and the bonding agents were applied and cured according to the manufacturer s instructions. The posterior composite was condensed and light-cured incrementally, in three horizontal layers. The samples were thermocycled for 200 cycles of 4 minutes each between 4.37 and 60 degrees centigrade, covered with fingernail varnish and sticky wax and immersed in 0.5% basic fuchsin dye for 12 hours. The specimens were split in two halves longitudinally in a mesiodistal direction in a hard tissues microtome and the degree of dye penetration was evaluated using an optical microscope. According to the results obtained we come to the conclusions that: 1. The use of Scotchbond 2 minimized but did not eliminate the microleakage around posterior composite resin restorations, in both types of cavities.(ABSTRACT TRUNCATED AT 250 WORDS)