In this study environmental scanning electron microscopy was used to observe the cracking of drying latex films below their glass-transition temperature. By controlling the relative humidity so that it decreases linearly with time, a critical level of humidity at which cracking occurs can be determined and this is measured as a function of film thickness. It was found that the cracking humidity decreases with increases in film thickness for thicknesses in the range of 30 to 100 mu m and then remains almost unchanged. A scaling a rimment can be used to fit the data very well and indicates that cracking occurs as soon as the entire film is consolidated into close packing.