Ultrafast dissociation dynamics of chloroiodomethane (CH2ICI) in the B band is studied by femtosecond timeresolved time-of-flight (TOE) mass spectrometry.Time-resolved TOF mass signal of parent ion (CH2IC1+) and main daughter ion (CH2Cl+) are obtained.The curve for the transient signal of CH2IC1+ is simple and can be well fitted by an exponential decay convoluted with a Gaussian function.The decay constant determined to be less than 35 fs reflects the lifetime of the B band.Significant substituent effects on photodissociation dynamics of CH2IC1 compared with CH3I are discussed.The dissociation time from the parent ion CH2ICl+ to the daughter ion CH2Cl+ is determined in the experiment.The optimized geometry of the ionic state of CH2ICl and the ionization energy are calculated for further analysis of the measurements.In addition,compared with the parent ion,a new decay component with time constant of ～596 fs is observed for CH2Cl+,and reasonable mechanisms are proposed for the explanation.