A novel method is presented to retrieve the average predissociation rate and the average rotational autoionization rate of a molecule by measuring the field ionization spectrum only, Both an intuitive picture and a quantitative description are derived for the competition between the three decay channels. field ionization. rotational autoionization, and predissociation. Field ionization spectra of various rotational states of ammonia are presented and fitted. From the fits we found that the average predissociation rate of the Rydberg series converging to the vibrationless N+ = 3 ionic state is about equal to the average rotational autoionization rate to the Rydberg series converging to the vibrationless N+ = 2 ionic state. namely 1 x 10(15)/n(3) s(-1).