In this paper, we present some theoretical results and we use modern electronic equipment to investigate experimentally rear-end collisions between two solitary waves. The surface position, water-particle velocity, and water pressure of the solitary waves are studied for surface waves in an irrotational water flow without an underlying current, with a (uniform) underlying current flowing in the same direction as the wave (“following” current), and a current flowing in the direction opposite to that of the wave (“opposing” current). The experiments involve a small leading wave that is overtaken by a large trailing wave, with a “compound” wave created when the waves overlap. The wave height, the water-particle velocity, and the water pressure are measured as functions of time using a wave gauge, an electromagnetic meter, and a pressure transducer, respectively. These measurements show that in all three scenarios the compound-wave amplitude decreases during rear-end collisions and reaches a minimum when the wave crests overlap. The measured water-particle velocity of a single solitary wave is also compared with that predicted by second- and third-order solutions of the governing equations, and the measured vertical distribution of pressure is compared with the theoretical vertical distribution of pressure with and without a current. We also study how the current direction (i.e., following or opposing) affects a solitary wave and present extensive results of water velocity and pressure for rear-end collisions between two solitary waves. For a following current, the wave velocity is greater, but the pressure is less than for a wave with a zero current, while for an opposing current the situation is reversed. This indicates that an underlying current affects the water-particle velocity field and that velocity and pressure are related in rear-end collisions. Thus, the water-particle velocity and pressure in the compound wave are greater than in the small wave and less than in the large wave.