This study evaluated the diametral tensile strength of type I glass ionomer cements and measured their tensile and shear bond strengths to dentin with and without conditioning. Four brands of glass ionomer cement and one brand of zinc phosphate cement were tested. Disks of each cement type were prepared and the 7-day diametral compression test for tension was conducted. The shear and tensile tests, with and without 10% polyacrylic acid dentin conditioning, were performed with an Instron testing machine. Two additional groups, one for each bond test, were prepared and conditioned with 90% trichloracetic acid. No significant differences of the diametral strength were found among the various glass ionomer cements. The comparison of means suggests that, except for trichloracetic acid dentin pretreatment, the conditioning is an effective promoter of shear strength and has no significant effect (p < 0.01) on the tensile bond strength. However, trichloracetic acid was found to be a most effective dentin conditioner for both shear and tensile bond strengths (p < 0.01).