She won the female
Dutch Chess Championship ten times (1937, 1939, 1946, 1948, 1950, 1952, 1954, 1956, 1958 and 1961). Heemskerk won a match against
Catharina Roodzant 4.5 : 0.5 in 1937, and lost a match to
Sonja Graf 0 : 4 in 1939 in Amsterdam. She took eighth place in
Women's World Chess Championship at Moscow 1950 (
Lyudmila Rudenko won), tied for second/third in Candidates Tournament at Moscow 1952 (
Elisabeth Bykova won), took ninth place in Candidates Tournament at Moscow 1955 (
Olga Rubtsova won), tied for 15–16th in Candidates Tournament at Vrnjacka Banja 1961 (
Nona Gaprindashvili won). She played in the
1st Women's Chess Olympiad at Emmen 1957 – but had to withdraw after only two days upon learning her father had died. In 1989, at the age of 70 she participated in the Moscow GMA qualifiers alongside strong male Grandmasters. Heemskerk was awarded the
Woman International Master (WIM) title in 1950, and the
Woman Grandmaster (WGM) title in 1977. She was invested as a Knight of the
Order of Orange-Nassau. In January 1940 Heemskerk married FIDE Master Willem Koomen{{cite journal ==References==