Transmission electron microscopy (TEM) observations reveal the presence of complex superstructures and remarkable phase separation in association with Na-ordering phenomenon in gamma-Na0.5CoO2. Resistivity and magnetization measurements indicate that three phase transitions at the temperatures of 25, 53 and 90 K, respectively, appear commonly in gamma-Na0.5CoO2 samples. Under a high pressure up to 10 kbar, the low-temperature transport properties show certain changes below the charge order transition; under an applied magnetic field of 7 T, phase transitions at around 25 and 53 K, proposed fundamentally in connection with alternations of magnetic structure and charge ordering maintain almost unchanged. (c) 2005 Elsevier B.V. All rights reserved.