Edward Schunk and Leon Marchlewski were to play a decisive part in the history of research on chemistry of chlorophyll and its derivatives. During only a few-year stay of Marchlewski in the Schunck laboratories in Kersal outside Manchester, a groundbreaking invention in the history of biochemistry took place: the close chemical relationship on the level of chlorophyll derivatives and haemoglobin was revealed. Also then, despite a considerable age difference between them, Schunk, who was at the end of his scientific career and Marchlewski just at the start, they became close friends. On the basis of the collection of original Schunk’s letters to Marchlewski, written in the years 1889–1902, we tried to reconstruct the character of their relationship in science, confronting topics raised in the letters with works published at the time. We sought to establish whether Schunk could have been inspired by research of his Polish colleague and, in turn, if remarks and suggestions of the Englishman could have influenced Marchlewski’s research programme. For better understanding the field of research of both scientists, we also shortly described the state of knowledge on chlorophylls and their derivatives at the turn of the 19th and 20th centuries.