Time variation of the density of Penning electrons produced by deexcitation of excited Ne atoms (mostly in the P-3(2) state) by polyatomic molecules (neo-C5H12, n-C6H14, and C-C6H12) is directly observed by the pulse-radiolysis microwave-cavity method. Analyses of the first-order growth of the electron density give the rate constants of total deexcitation, which has been divided, with the aid of measurements for admixtures with Ar or Xe, into the rate constant for the Penning ionization and that for the molecular dissociation. The ionization efficiencies are 0.93, 0.80, and 0.91 for neo-C5H12, n-C6H14, and C-C6H12, respectively.