In this paper we report a technique for generating a large, homogeneous volume of ionized gas and for making time‐resolved measurements of its electron density. We also report absolute photoionization coefficients for a number of molecules. These coefficients will allow calculation of peak electron densities obtained in other ionization‐related experiments. In our technique, the beam from a rare‐gas halogen laser photoionizes organic molecules that have been seeded into a buffer gas. Ionization is accomplished by the absorption of two photons. We have analyzed the electron densities produced by photoionization with a microwave interferometer that measures the electron density in the ionized gas with a time‐resolution of a few nanoseconds. Some of the organic molecules photoionized in this work produce electron‐ion pairs with an efficiency more than five orders of magnitude higher than commonly used laboratory ionization sources such as ArF laser photoionization of NO. We have also studied the effect of attaching gases on the ionized mixtures and have demonstrated that addition of as little as 10 Torr of SF6can reduce the peak electron density observed by a factor of 103.