An alcohol (ROH) solution of p-haloaniline, p-NH2C6H4X (X = Cl, Br, I), was introduced into vacuum as a continuous liquid flow (liquid beam) and was irradiated with a UV nanosecond laser having a pulse width of similar to 5 ns or a UV femtosecond laser having a pulse width of similar to 200 fs. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by a time-of-flight mass spectrometer. The mass spectrum of ions ejected from the liquid beam exhibits peaks assignable to H(-NHC6H4-)(n)H+, H(-NHC6H4-)(n)OR+, and H(-NHC6H4-)(n)X+. The spectral features change significantly when Che pulse width of the excitation laser is elongated from the femtosecond to the nanosecond range : (1) significant reduction of the H(-NHC6H4),Brf intensities and (2) increase of the degree of polymerization in H(-NHC6H4-)(n)H+ and H(-NHC6H4)(n)OC2H5+. These findings indicate that a sequential polymerization of p-NH2C6H4X is driven by photoabsorption. The degree of polymerization is explained in terms of rate equations of the polymerization.