A series of small-scale experiments on vented methane-air explosions are carried out in a 22 L spherical vented vessel with small vent areas. The experiment is conducted under both constrained and unconstrained circumstances where the dimensionless vent ratios are different. A discussion of the various characteristics of the explosion is presented. Under the constrained circumstances, reduced dimensionless vent ratios can lead to an increase in the maximum explosion venting pressure and in the maximum explosion rising rate, called nonbalanced vented explosions. However, when the dimensionless vent ratios are higher than a certain value, the maximum explosion venting pressure and maximum explosion rising rate do not change, which is called as balanced vented explosions. When the dimensionless vent ratios are between 0.009 and 0.025, it has a remarkable effect. When the dimensionless vent ratio is between 0.00169 and 0.009, it doesn't affect the explosion pressure. However, a very high explosion venting pressure appears. Moreover, the pressure is close to the maximum explosion pressure in a closed vessel. When the dimensionless vent ratios are lower than 0.00169, the maximum explosion venting pressure is higher than the maximum explosion pressure in a confined space. Nevertheless, under the unconstrained circumstances, the maximum explosion venting pressure is always lower than the maximum enclosed explosion pressure despite of different dimensionless vent ratios. (C) 2017 American Institute of Chemical Engineers