Microwave irradiation is proposed as a pretreatment method to accelerate the conventional steam distillation of rosemary essential oil. Microwave extraction methods are based on the capacity of radiation to break the oil containing glands, allowing faster extraction rates. An analytical method is developed to quantify the fraction of essential oil inside and outside trichomes. This method is used to evaluate the effect of microwaves on the fraction of oil released outside of trichome glands. Steam distillation rates are measured after using different Microwave pretreatment energies, and the influence of liquid moisture released from plant cells is also analyzed.