Dame Kathleen Lonsdale DBE FRS (née Yardley; 28 January 1903 – 1 April 1971) was an Irish crystallographer, pacifist, and prison reform activist.

She proved, in 1929, that the benzene ring is flat by using X-ray diffraction methods to elucidate the structure of hexamethylbenzene.

[17] As the unrest in Ireland became more severe Kathleen's mother separated from her father and took the rest of the family to England.

[21] In 1924 she joined the crystallography research team headed by William Henry Bragg at the Royal Institution.

[22] From 1929 to 1934, she started a family and largely stayed at home while continuing her work calculating structure factors.

Lonsdale was one of the first two women elected a Fellow of the Royal Society (FRS) in 1945[1] (the other was the biochemist Marjory Stephenson).

As a keen table tennis player, Lonsdale made use of ping pong balls to demonstrate the molecular structure to her students.

[39] "What I was not prepared for was the general insanity of an administrative system in which lip service is paid to the idea of segregation and the ideal of reform, when in practice the opportunities for contamination and infection are innumerable, and those responsible for re-education practically nil"[40] In 1953, at the Yearly Meeting of the British Quakers, she delivered the keynote Swarthmore Lecture, under the title Removing the Causes of War.

A self-identified Christian pacifist,[41] she wrote about peaceful dialogue and was appointed the first secretary of Churches' Council of Healing by the Archbishop of Canterbury William Temple.