High pressure pipeline transportation has been an established technology for economically transporting large amounts of CO

2

. However

there are still issues and associated risks that have to be effectively addressed and adequately understood. It is well known that a strong Joule-Thomson Cooling effect can occur when pressurized CO

2

flows through a choke valve. Thus

to investigate the choking characteristics especially the temperature drop of high pressure CO

2

a new laboratory scale experimental setup (total length of 14.85 m and the inner diameter of 15 mm) was constructed. Steady choked flow and transient choked flow tests were carried out respectively for pressurized CO

2

in various initial phases. The phase transitions and temperature drop characteristics were then studied following the choked flow and the results show that the phase transitions in steady choked flow differs significantly from that in transient choked flow. For transient choked flow of various initial phases

all the flows downstream would transfer from single phase to gas-liquid two-phase flow. Furthermore

the effect of water on transient choked flow of supercrit

ical CO

2

pipeline was investigated

and the phenomena of solid particles deposition was captured which was paramount importance of ensuring the safety operation of CO

2

pipelines when throttling by the choke valves.