Individual ultrasound contrast agent microbubbles (BR14) were characterized acoustically. The bubbles were excited at a frequency of 2 MHz and at peak-negative pressure amplitudes of 60 and 100 kPa. By measuring the transmit and receive transfer functions of both the transmit and receive transducers, echoes of individual bubbles were recorded quantitatively and compared to simulated data. At 100 kPa driving pressure, a second harmonic response was observed for bubbles with a size close to their resonance size. Power spectra were derived from the echo waveforms of bubbles of different sizes. These spectra were in good agreement with those calculated from a Rayleigh–Plesset-type model, incorporating the viscoelastic properties of the phospholipid shell. Small bubbles excited below their resonance frequency have a response dominated by the characteristics of their phospholipid shell, whereas larger bubbles, excited above resonance, have a response identical to those of uncoated bubbles of similar size.