diff --git a/changelog/unreleased/add-write-mutexes.md b/changelog/unreleased/add-write-mutexes.md
new file mode 100644
index 000000000..ec16003a1
--- /dev/null
+++ b/changelog/unreleased/add-write-mutexes.md
@@ -0,0 +1,5 @@
+Bugfix: Add write mutexes
+
+Concurrent account or groups writes would corrupt the json file on disk, because the different goroutines would be treated as a single thread from the os. We introduce a mutex for account and group file writes each. This locks the update frequency for all accounts/groups and could be further improved by using a concurrent map of mutexes with a mutex per account / group. PR welcome.
+
+https://github.com/owncloud/ocis-accounts/pull/71
\ No newline at end of file
diff --git a/pkg/service/v0/accounts.go b/pkg/service/v0/accounts.go
index a72179451..6a3e8ec2c 100644
--- a/pkg/service/v0/accounts.go
+++ b/pkg/service/v0/accounts.go
@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"sync"
 	"time"
 
 	fieldmask_utils "github.com/mennanov/fieldmask-utils"
@@ -84,6 +85,8 @@ func (s Service) loadAccount(id string, a *proto.Account) (err error) {
 	return
 }
 
+var accountMutex sync.Mutex
+
 func (s Service) writeAccount(a *proto.Account) (err error) {
 
 	// leave only the group id
@@ -95,6 +98,9 @@ func (s Service) writeAccount(a *proto.Account) (err error) {
 	}
 
 	path := filepath.Join(s.Config.Server.AccountsDataPath, "accounts", a.Id)
+
+	accountMutex.Lock()
+	defer accountMutex.Unlock()
 	if err = ioutil.WriteFile(path, bytes, 0600); err != nil {
 		return merrors.InternalServerError(s.id, "could not write account: %v", err.Error())
 	}
diff --git a/pkg/service/v0/groups.go b/pkg/service/v0/groups.go
index 666d8608d..150dc7d60 100644
--- a/pkg/service/v0/groups.go
+++ b/pkg/service/v0/groups.go
@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sync"
 
 	"github.com/CiscoM31/godata"
 	"github.com/blevesearch/bleve"
@@ -66,6 +67,8 @@ func (s Service) loadGroup(id string, g *proto.Group) (err error) {
 	return
 }
 
+var groupMutex sync.Mutex
+
 func (s Service) writeGroup(g *proto.Group) (err error) {
 
 	// leave only the member id
@@ -77,6 +80,9 @@ func (s Service) writeGroup(g *proto.Group) (err error) {
 	}
 
 	path := filepath.Join(s.Config.Server.AccountsDataPath, "groups", g.Id)
+
+	groupMutex.Lock()
+	defer groupMutex.Unlock()
 	if err = ioutil.WriteFile(path, bytes, 0600); err != nil {
 		return merrors.InternalServerError(s.id, "could not write group: %v", err.Error())
 	}