The gas phase photodissociation spectra of four protonated fi-diketones were obtained and compared with the absorption spectra of the corresponding ions in solution. Protonated 2,4-pentanedione was observed to undergo the photodissociation process [CS&O2]++hv--, [CH,CO]'+C,&O with a Amax at 276* 10 nm compared with a solution absorption maximum at 286nm. Protonated 2,4-hexanedione was observed to undergo the photodissociation processes [C,H,,O,l'
[C~H,,O~]'+hu-[C,H,CO]'+C,&O
with a Amax at 279k 10 nm compared with a solution absorption maximum at 288 nm. Protonated 3-methyl-2,4-pentanedione was observed to undergo the photodissociation process [C6Hl10,]'+hv-,[CH3CO]'+C4~0
with a Amax at 295~k10 nm compared with a solution absorption maximum at 305 nm. Protonated l,l,l-trifluoro-2,4-pentanedione was observed to undergo the photodissociation process [C,&F30,]'+hv~CF3H+[C4HsO~]+ with a h a x at 273& 10 nm compared with a solution absorption maximum at 288 nm. The [CH,CO]' and [C,H,CO]+ produced photochemically with the first three ions react to regenerate the protonated P-diketone leading to a photostationary state. Photodissociation of the protonated alkyl P-diketones is believed to occur from the protonated keto form, whereas photodissociation of protonated l,l,l-Mfluoro-2,4-pentanedione is believed to occur from the protonated enol form. Mechanisms for the observed photodissociation processes are proposed and comparisons with results from related techniques are presented.