Pump-probe experiments with subfemtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser. By measuring the delay between the two pulses with an angular streaking diagnostic, we characterize the group velocity of the X-ray free-electron laser and show control of the pulse delay down to 270 as. We confirm the application of this technique to a pump-probe measurement in core-ionized para-aminophenol. These results reveal the ability to perform pump-probe experiments with subfemtosecond resolution and atomic site specificity.|Researchers have demonstrated the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser and conducted pump-probe experiments in core-ionized molecules.