Knowledge on cavitation bubble size distribution, ambient radius of bubbles is of interest for many applications that include therapeutic and diagnostic medicine. It however becomes a hard quest when increasing the ultrasonic frequency, when direct observation of bubble dynamics is no longer possible. A powerful experimental approach1 to measure the size distribution of bubbles active in sonoluminescence and/or sonochemistry is a technique based on pulsed ultrasound and sonoluminescence emission. While it is an accepted technique, it is still lacking an understanding of the effect of various experimental parameters, including the duration of the pulse on-time, the nature of the dissolved gas, the presence of a gas flow rate, etc. The present work2, focusing on Ar-saturated water sonicated at 362 kHz, shows that increasing the pulse on-time leads to the measurement of coalesced bubbles. Reducing the on-time to a minimum and/or adding SDS to water allows to reducing coalescence so that natural active cavitation bubble sizes can be measured. A radius of 2.9-3.0 µm is obtained in Ar-saturated water at 362 kHz. The effects of acoustic power and possible formation of a standing-wave on coalescence and measured bubble sizes are discussed. Conditions optimized with Ar are then used to derive ambient radii of cavitation bubbles in water saturated with He, Ar, Xe, O2, N2 and air: 3.0 µm for Ar, 1.2 µm for He, 3.1 µm for Xe, 2.8 µm for O2, around 1 µm for N2 and air3. If the pulse on-time is increased, bubble coalescence occurs, the extent of which is rather limited for Ar but extremely high for He or N2. 1 Lee, Ashokkumar, Kentish, Grieser, J. Am. Chem. Soc., 127 (2005) 16810-16811. 2 Pflieger, Bertolo, Gravier, Nikitenko, Ashokkumar, Ultrason. Sonochem., 2021, 73, 105532 3 Pflieger, Audiger, Nikitenko, Ashokkumar, Ultrason. Sonochem., 2021, 73, 105537