Some 15 inmates at Bungoma GK prison sat their KCPE exam on Tuesday.
They are among 17 who registered for the examinations but two were released before they began.
Prison inspector Stephen Githinji said the candidates were well prepared and is optimistic they will pass with flying colours.
"We are definitely sure that these inmates have been well prepared and we are sure when the results come out they will have performed well as this class is much brighter than the previous one we've had here," he said.
Among those sitting the exam was Joseph Makhanu, jailed for defiling a minor.
The 65-year-old is the oldest candidate in the facility and said he opted to do the exam because his father was poor and could not afford to take him to school.
"I was arrested and jailed for 10 years and I have now completed seven. I was accused of defiling a 21-year-old girl who was my friend," he said.
The grandfather of four claimed he was wrongly accused and he could not defend himself.
Makhanu, who comes from Kiminini, Trans Nzoia county, hopes to become a doctor if he gets the opportunity to go on with learning.
John Waliaula, 19, is serving a two-year sentence for theft.
He said he dropped out of school in class seven after his parents died and being in prison has brought back the zeal to study.
"Since I was a child I have always had a dream of being a doctor to treat people. God willing, if I complete my sentence next year and get a sponsor, I can further my studies," he said.
Waliaula asked fellow inmates to ensure they study so when they get out of prison, they become responsible members of society.
"I am hoping to get 375 marks and above and join a national school so that I may be able to help my siblings and family members back at home," he said.
About 45,000 candidates will be sitting the KCPE exams in Bungoma county for the next week.
Governor Wycliffe Wangamati who witnessed the start of the exams at Kanduyi DEB where he did his CPE in 1983 wished the candidates good luck.
He said that his government is making arrangements to have two streams at Lugulu and Kamusinga national schools that will enroll Bungoma students.
County Kuppet chair Denison Baraza asked politicians to refrain from chaos as it will make the environment hostile for candidates to sit exams.
In Meru GK Prison, 23 inmates sat the exams.
Welfare officer Enock Momanyi said they hope to produce the highest marks in the county.
He said they are aiming for not less than 350 marks, 36 marks more than the highest score last year.
Momanyi said among challenges faced is squeezed learning spaces.
"We registered a total of 33 KCPE candidates. Eight women, but only one turned up. Seven who were released did not show up. Three men who were also released did not show up," he said.
Isaiah Rukunga, 40, is among candidates who sat the first mathematics paper.
"There is democracy even inside prison. The only freedom that is restricted is that of movement," he said.
Governor Kiraitu Murungi said sufficient security has been deployed for the exam period.
"I pray, encourage and wish success all pupils and students in Meru and Kenya at large. I believe you're hardworking will pay," Kiraitu said.
Some 30,881 candidates are registered to sit KCPE in Meru. The exams took place without hitches.