Two pseudotetrahedral cobalt(II) complexes exhibiting slow magnetic relaxation under an applied direct-current field are investigated. The weak easy-plane anisotropy is accurately determined by high-field/high-frequency electron paramagnetic resonance spectroscopy as D = 2.57 cm(-1) and E = 0.82 cm(-1) for 1 and D = 5.56 cm(-1) and E = 1.05 cm(-1) for 2. In addition, hysteresis loops are observed for the two compounds at very low temperatures.