An attempt has been made to explain the properties of the ``plasma'' of fluorescent lamps in considerable detail, using the ambipolar diffusion theory of Schottky, modified to include ionization by multiple electron impacts. Ionization is assumed to occur by electron collisions with excited and metastable atoms. The cross section for this process is one of two inaccurately known parameters, which we treat as adjustable constants, occurring in the calculations. The other is the effective diffusion constant for quanta of the resonance line, in the presence of a high‐temperature electron gas.Qualitative agreement of the foregoing theory with experiment is demonstrated for all the derived quantities as functions of the several parameters of the discharge. The magnitudes of the calculated electron temperature, electric field, total light output, and efficiency are also in sufficiently good agreement with experimental values for the lamps of greatest commercial interest to permit extrapolations to be made with reasonable confidence. Optimum agreement of theory with experiment for T‐12 lamps is obtained using a mean life time of ultraviolet photons in the discharge 3.6 times that employed by Kenty for diffusion in un‐ionized vapor, and with a value 3.3 for the ratio of cross section for ionization of excited state to that of the ground state.