The effects of urbanization on the temperature characteristics of a tropical city are examined. Twenty-year records of maximum, minimum and mean temperatures were extracted from four established meteorological stations in the Ibadan area. Three stations are within the urban region and the remaining one, which serves as the control station, is situated in a rural environment, outside the city. Analysis shows that the urban heat island effect of the city is clearly reflected in the magnitudes of night-time minimum and mean temperatures, whereas the day-time maximum temperature is slightly lower, in the city. There is no clear evidence of any change in the urban temperature excess over time. The heat island effects of the city are more noticeable towards the end of the dry season and in the early part of the rainy season. The mean heat island, as reflected by the night-time minimum temperature, is about 1.0 K during the dry season, and 0.5 K during the rainy season.